It's designed to be used with a MIDI keyboard. It basically just transforms a particular MIDI note (which can be learned) into another random note. The little keyboard on the GUI can be used to determine what notes are generated.
To use it, you should place Ear Trainer into the VSTi Rack along with another VSTi to make the sound (Kontakt, MDA piano, ect.)
Create two MIDI tracks. On one, set the input to your MIDI keyboard and the output Ear Trainer. On the second MIDI track, set the input to Ear Trainer and the output to whatever VSTi you want to use to make the sound. Lastly, learn the notes for the play and repeat buttons!
Download
http://www.audiofront.net/earTrainer.vst3