To add a VST effect to one of your tracks, click the Fx button in the track control panel on the left of the track. In the left hand panel of the dialog that opens you should see a sub-heading VST Effects. Any valid VST plugins you have installed will be listed underneath this sub-heading. If there are no items listed here, it means that you either do not have any VST plugins installed, or you have not configured your VST plugin path. You must tell MixPad where to look for your VST plugins. You can do this by going to the VSTs tab of MixPad's Options dialog.
To add MIDI playback or monitoring to a track, open the track options dialog by clicking on the small spanner icon on the track controls. In the dialog that opens you should see a MIDI Playback Options option group. Select the radio button that says "Play MIDI using VST instrument" and then select which instrument you would like to use.