1) Add your VST instruement (menu: Add Channel | Add new instrument channel)
2) Create a MIDI track and point the output to the new VSTi channel.
3) Look at the piano roll for your new MIDI track. If you want to hear it real-time, click the 'Live' button and then click the piano keys to hear. Then you can start to program your MIDI track.
4) Save
5) Click the 'Play' button.
Viola...VSTi.