How To Select A Virtual Instrument 
Mixcraft is designed to work with virtual instruments and synthesizers.  

To select a virtual instrument, simply create a Virtual Instrument Track by clicking Track, followed by clicking Insert Track, followed by clicking Virtual Instrument Track.

Click on the instrument icon:

This brings up the instrument dialog:

Select a category and a preset.

Close the window by clicking the [x] on the top right corner, if you'd like.  Make sure that the track is selected and then go to your MIDI keyboard and start playing!

More details on choosing a Virtual Instrument.