Maybe I just don't get it, as a matter of fact, I'm sure that's it... I'll set up the situ... I am recording an original tune and want to lay down a bass track, the keyboard I have does not really have an appropriate (read: pleasing) bass patch for this song... I have opened up Studio Instruments and find some nice sounds, but I'm not sure how to play them into my song... 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, all the stuff you can learn easily enough just playing around with the VI. So how do I play a bass line for my particular tune with Studio Instruments, can I trigger the sounds with midi from my keyboard? Or is it strictly a pattern based system?

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.

You can also draw the notes into the piano roll if you'd rather work like that instead of playing it on your keyboard. Or you can play the part on your keyboard and then edit your performance in the piano roll later. Or if like me you're a crap keyboard player, you can simply bang the rhythm of the part on one note of your keyboard and then go into the piano roll view and move the notes into the correct pitches. Loads of different ways to do this!

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.

Great suggestions, I'll give it a try

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.

I do a lot of this and I don't use my keyboard for my bass sounds in the final mixes. I go get samples in Reason or use Kontakt, or use the Cakewalk basses.
My method is simple
1. I use my keyboard to play and create the bass part on a midi track
2. Open up and insert a Cakewalk Bass
3. Copy and paste the midi track into the Cakewalk Bass track you created (no need even listening to the pre-loaded stuff it usually doesn't relate to your original music...I can see using those loops to practice or flesh out ideas but once you actually start writing...create a bass line) 
4. Mute the original midi bass track so all you hear is the Bass track you pasted to...it's now a track you can use in your final mix. I like to save and keep the original, muted midi file right there in the song in case I want to change them later...then I just erase and re-paste the new part back into the Cakewalk Bass track) 
Voila...your part should play fine and you can mess with the tones and pickup selection using the interface the Bass has...(plus any EQ, or compression from the FX bin inserted in the track.
 
Have fun
Jason216

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. 
 

Cactus - I agree and have been using Cakewalk since DOS lol...It's really handy to slow your track down to do stuff you couldn't play otherwise...I don't even start messing with any audio tracks until all my midi stuff is done...That way I can get the music done at whichever tempo I need in order to play it...then do all soft-synth inserting, and any audio stuff last like guitars, vocals etc...For me..this works fine
Jason216

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.