Let's start with the trackers. I've used Magix Music Maker myself and as far as I can recall it has no MIDI capability. Well, not the very old versions I last saw anyway.
I don't know what you mean when you say 'MMM doesn't have a keyboard'. If you mean an on-screen virtual layout is what you're after, then try using Cantabile:
http://www.cantabilesoftware.com/. It's free, supports VST and VSTi and has a simple sequencer. Other free MIDI sequencers can be Googled, though I've not tried them:
http://www.google.co.uk/search?hl=en&q= ... ncer&meta=.
Some Commodore 64-style virtual instruments patch in to the MIDI sequencer and can be played through the PC (simply by installing or manually copying the DLL file to the sequencer's plug-in folder). They can be found here:
http://www.google.co.uk/search?q=commod ... GB284&sa=2.
Connecting your external keyboard to the sequencer is simplicity itself. Get yourself a PC MIDI cable and plug the 15 pin plug into your computer's joystick socket. Then plug the two In and Out 5 pin DIN plugs into your keyboard's MIDI ports. And if you want to record the sound of the keyboard itself, run a standard audio cable from your keyboard's audio or low-impedence headphone out to the line in on your soundcard.
Remember that just the line out carries the actual audio, whilst MIDI only concerns itself with control data. In your sequencer all you need to do is activate your MIDI controller in the relevant dialog box, activate the recording channel in MIDI mode and you're away. Just like that.
Of course another way to do the MIDI thing in the 64's orbit is to buy a MSSIAH cartridge for the 64 itself (see root page of this very site) or try to strike it lucky on Ebay with this especially nice MIDI interface (
http://cgi.ebay.co.uk/Rare-Sequential-c ... 286.c0.m14). Then just download something like Sonus Super Sequencer and port the disk image to a floppy disk or load directly into the 64 using one transfer device or another.
Instant Music thread:
viewtopic.php?f=3&t=6949
My general C64 page, with links to what I've mentioned:
http://homepage.ntlworld.com/michael.br ... Record.htm
Some of my own music, with the C64 often involved in one way or another:
http://www.concept-single.net/
http://www.youtube.com/watch?v=DjbBw7ctP-0
http://www.youtube.com/watch?v=BvVc-oHW ... re=channel
http://www.youtube.com/watch?v=wY2cZrrW ... re=channel
http://www.youtube.com/watch?v=VTwFLsaM ... re=channel