As soundtrack being designed for Mac.......so is Metro.......there isn't a PC version out there.
The only thing that may seem to hinder Metro in development is loyality to OS9 users which I have never seen anywhere else.......and I can't think of anything I need in OSX that isn't there so it isn't really a negative reason not to go with it. You can also demo the different versions for 25 minutes at a time to try it out.
The thing you need is the connection between the mac/metro and the triton - the midi interface.
Something from this page perhaps?
http://www.audiomidi.com/master.cfm?SID=26
The Edirol UM1X looks like a good choice as it comes with the necessary cables and assuming you only have one midi device you won't need more midi in and outs.
You would connect the triton OUT to the IN on the interface - this allows you to record notes that you hit on the synth and allow it to be inputted/recorded into the sequencer.
The OUT on the interface would go into the IN on the synth - when playingback in the sequencer this allows the sequencer to trigger the sounds on your synth so you hear them instead of using something like the built in quicktime instruments (once outputs have been assigned to the triton in the sequencer).
I got someone to download metro and even without there own synth, they were happily constructing notes and beats reasonably quickly with the pencil tool....though they'd never used a sequencer before.