I only know about Logic under OSX
Logic in OSX only plays plugs that are Audio Units (apple's pain in the ass plug-in format). So VSTs are out, unless you have a program that "wraps" the VSTs or one that converts VSTs to AU
If you have AU plugs then all you do to load one is to go to the track mixer window, go to an "audio instrument" channel strip, and right above where the channel strip reads output 1-2, click and hold the mouse, a pop-up should appear allowing you to choose what plug you want to use. you should be able to use your midi keyboard to play that instrument (so long as you have loaded the keyboard into Logic's midi controller preferences.)
BTW do you have a copy of Logic6 on a PC? I thought version 6 was only made for MAC...
And lastly, even with all my soft synths (Logic's bundles stuff, Reaktor4.something, Reason2.5, etc) I would love an MPC...