Hi,
I have used Poly800 as MIDI controller with software for longer time.
But it is limited (yet mentioned no-velocity, no sustain-pedal...).
If you have something like
http://www.musiciansbuy.com/mmMBCOM/images/roland_UM1SX.jpg , you have
to connect MIDI cable from POLYs MIDI OUT to Comp`s MIDI IN.
POLYs keyboard transmitts on MIDI channel 1, so you have to set input
channel for recorded track in software on Ch1, or OMNI On - in this
case track will recieve data from all 16 MIDI channels.
May be, you have some different cable - e.g.
http://images.amazon.com/images/P/B00006343G.01._SCLZZZZZZZ_.jpg . You
have to plug DIN connector marked as MIDI IN to POLYs MIDI OUT (i hope
cable-producents marks it uniformly, like on my cable...:-) )
Jozef