In this case, MixPad supports Virtual Studio Technology (VST) plugins. You can use these plugins to enhance MixPad's capabilities. You can use effect plugins to add an effect to one of your tracks, or you can use instrument plugins to synthesize playback of MIDI files. Other plugins may provide some type of visual feedback of the audio signal.
Some, VST plugins are available from a wide variety of sources - some are free, while others require licences. A good place to start your search is here: http://www.kvraudio.com
Could be possible that those plugins are not compatible.
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.