Studio Instruments

Yes, you can trigger SI from your MIDI keyboard. I rarely, if ever, use the included patterns.
Just select the keyboard as the input in the MIDI track. Your keyboard(or the MIDI port it is connected to) must be enabled in Edit>MIDI Devices. Also, input echo must be turned on in the track for you to hear what you are playing.
SI Bass has a limited range, so not all keys will sound a note.

tindog13
... the "patterns" are of no use to me, can't really even imagine them ever being useful, but that's what the tutorials I find talk about mostly, how to arrange the patterns and save and tune...

Same here.  I gave up on patterns years ago.  I don't see how it would be possible to find the exact thing you were looking for in less time than it would take to make it yourself.

Midi is a mystery to me, no matter how much I read on it and play with it... I can choose my keyboard as input, but I don't see where or how to choose the SI sound banks to trigger.

You need to insert a soft synth.  
Then you set the input of a midi track to receive your controller, you assign the output of the midi track to the soft synth, There are different variations on midi tracks and how they relate to the soft synth so you will be best off reading the tutorials and help files about the subject. 
This is a simple process once you get the jist of it. 
Patterns are for people who do not play an instrument to use, if you know how to play a bass line on the keyboard then you will have no problem. With midi you can even slow the tempo down at first and play to a click track,
My self I will record my scratch vocals and a guitar using a simple drum groove or the metronome. Then I add the bass. I used to use midi bass but switched to real. My first midi bass track was on an Atari 35 years ago. Nothing much has changed since about the process other than VST soft synths instead of outboard gear. 
 

Even if I start a song with audio, I can still cheat playing a midi part with the slower tempo trick, Just mute all audio so it doesn't play. But make damn sure you remember your original tempo or it all goes out of sync. Worst case is you redo the audio. 
I have 100's of my old midi backing tracks I made myself I'm re-doing to real bass etc. I listen to the originals and wonder why nobody threw tomatoes at me for boring sound quality. Back then it sounded great! Why is that? 
 
But I do like you when I can and get the song going with midi first. So much easier to edit and change the arrangements, keys and tempo. 

Tindog13, one of the reasons I do my midi work that way is I had latency issues when using the soft-synths to play the parts I'm creating. Using the keyboard to create the midi parts then assigning things later to a soft-synth gets rid of that problem for me. Remember...don't get impatient to hear your final results while your creating. One of the joys of midi is the ability to change your sounds, even 20 years later, to the current available soft-synths of the day...truly cool...You will develop your "go to" sounds after a while and things will get faster once you know what they will sound like in the finished product. 
 
On bending the bass note...This is something I do, and what the loops available to you can provide...I have listened to all the bass loops that come with Cakewalk and have written down the ones that do cool little flourishes that are hard to re-create on keyboards, whether you have a mod wheel or not, somethings are just hard to do sooo... I add a 2nd Cakewalk bass into the mix just to find the add-ons I want to use...example...say I'm at the end of a bass part on the 4th or eight bar where I want a little bass thingy...well I go to the bass loop library and find the "specific" little flourish and edit it out (change the key to fit your song if you need to)...now I have this cool little bass thingy I just add into the original part. I do this with drums as well...You really can't do fills on a keyboard and get them to sound like a real drummer so I never do fills. I'll find a fill that sounds cool and sounds like a real drummer did it and just cut that fill out, that way I get to use my hi hat, kick and snare that I created for the main parts, and just add the things that you can't do well on a keyboard. I know this is time consuming at first 'till you get to know your loop libraries, but it's real effective in making things sound real. I occasionally will use an Acid loop or some other file but you have to EQ those to sound like the Cakewalk bass parts and sometimes that's hard to do...but if you're only using the Cakewalk bass soft-synth only...you won't have that issue. 
 
 
Hey Cactus - I have had the ..."Oh Sh**...what was the original tempo...?" thing happen to me before, especially when I bring the midi tracks into another program so I started naming all my tunes thusly... Stairway To Heaven b104... the small "b" is like a quarter note and the number is the tempo used...You'll never have that happen again.
 
Keep writing
Jason216

Tindog, do you have an interface, or are you using your computer's built-in sound card?
When recording MIDI/softsynths live, you are entirely at the mercy of your sound cards performance.
The MIDI data is recorded on time, but you won't hear the sound until your sound card/interface has processed the audio output of the synth.
 
Does your keyboard have the ability to change the MIDI channel it is transmitting on? If so, you can use this to your advantage.
Give us some details on your setup. It looks like you may be fighting some basic stuff, and we can help you better if we know what you are working with. The best approach is to list your system specs in your signature, as you see in many of our posts. That way, it is always available.

I'm using the Presonus Audiobox USB, and yes, I'm sure the keyboard has that capability... it's a Casio Privia Px-300, with the GM sound bank.