Hi Rain, looks like you might be making some progress but I'll take a punt to help you out.
As you've deduced you need 2 midi leads, 1 goes from the out of the mm6 to the in of the 144 and the in keyboard to the out of the 144. You also need a pair of jack leads to go from the main audio outs on the yamaha in to the line inputs on the 144. Finally you need to plug the output of the tascam in to your monitors / amplifier.
That's the easy bit. Now you have to make some decisions. Do you want to monitor 'directly' through your soundcard or are you happy that your computer is beefy enough to do realtime monitoring. (I'm going to assume the latter. ;-))
First open Sonar and create a midi track, set the input and output to the 144, when you play the key board you should see midi activity in the meter and the little icon on the toolbar. On the 144 turn the monitor mix knob to 'input' and turn up the 2 gains until you can hear the signal from the yamaha ok. (generally the yamaha volume should be turned right up)
Now, back in Sonar, create an audio track (and while you're at it a buss, which you'll call 'master' route the output of this buss to the 144 stereo out put) set the input of the track to the tascam stereo input and the output to 'master' arm the track to record and continue playing the keyboard. You should now see the audio meter on your newly created audio track dancing around. Pick a loud patch on the yamaha and jam along whilst adjusting the input gain on the tascam so the peaks of your audio are dancing around -18 on the meter in Sonar.
Now's the fun bit, on the tascam turn the monitor mix dial from 'input' to 'computer' and switch on 'input echo' on your audio track (and disarm it from record). Now the audio that's coming in is being processed by sonar and put back out via your master buss. If there's a noticable delay between pressing the key and hearing the sound go to 'options / audio' and reduce your latency by setting a smaller buffer.
What you've got now is midi control and audio monitoring. You should be able to arm your midi track and record your part. You can then tweak your midi around to make sure it's perfect, then arm the audio track and record the output from the synth at your leisure.
You can also control soft synths using the midi you've recorded, insert a new synth using the synth rack, and then change the output of your pre-recorded midi track to the synth.
Your system is about as simple as it gets so spend a bit of time digesting this, think about where the signals are flowing and where you're able to apply control to them and you should be able to scale this up to more complicated set ups.
As was already mentioned, MIDI is not Audio. Get your head around this concept and it'll all drop in to place.
Child