simple, go to your group... select a pad then hit SHIFT + BROWSE to go to your Modules setup... under "TYPE" use the selector knob (1st one under 1st display) to go to "PLUGIN" then choose the plugin using your 5th knob as can be seen in the second display....
hope that helps
that's how you do it without your mouse or keyboard lol... if you want to use the mouse just click your first modules drop down (the number 1 box next to the sound tab) and go to Plugins
if you hit the fifth button (located above the second screen; should be the name of the VST, for me it's MASSIVE) it'll open up the gui controller for the vst and you can even assign contollers for the vst...
youtube watch?v=vsFA-b9qi8k