You realize that you'll need a microphone, right?
You need to load the drivers for
the FastTrack Pro before you do anything else. Note that the latest Avid drivers are buggy as hell. Try to find the older ones from mAudio. They're available on line. If you can't find them, PM me.
Once you've loaded the drivers,
plug in the FastTrack Pro via a USB connection. Do NOT go through a USB hub.
You can chain all the MIDI instruments (including the workstation) together -- OUT (or THROUGH if the instrument has it) of one to IN of the next, with the final IN going to the OUT of the FastTrack Pro. If the instruments don't have THROUGH (or THRU) MIDI ports, you'll have to configure each to pass MIDI commands. This is not, however, the best way to do it as you may run into MIDI latency issues -- the extra time each instrument adds as it passes the commands to the next instrument. You're better off getting a separate USB MIDI interface with multiple ports, as it will eliminate latency issues and makes it unnecessary to configure each instrument for MIDI THROUGH.
MIDI doesn't transmit audio -- it's just a command protocol that tells the MIDI instrument which note to play, for how long, on which channel, etc. To get the audio in, you'll need to patch the audio OUT of each instrument to the FastTrack Pro. The FTP can only handle two audio inputs at a time, and it has limited ability to do that. You're going to have to get an audio mixer to mix the output of your synths and your microphone.
I've been working on a project that uses a lot of old synths. My solution was to get a FastTrack
Ultra 8R, which has 8 separate audio inputs. That's still not enough for the synths and samplers that I use, so I have a couple of mixers as well. It's really important, though, to record each synth to a separate track in your DAW (Cubase, in your case), or you'll have problems mixing.