Adding Virtual Instrument Clips
You can add Virtual Instrument Clips in several ways.

Create a blank Virtual Instrument Clip.
Double click a Virtual Instrument Track or right click on a Virtual Instrument Track and click Add Instrument Clip to create a blank Virtual Instrument Clip. 

Record a Virtual Instrument Clip.
Arm a Virtual Instrument Track and record.  Use your MIDI keyboard to play back notes and click Stop on the Master Bar when you are finished.

Import a MIDI file.
You can right click on a Virtual Instrument Track and select Add Sound...  Select a MIDI file (.MID) and click Open.  (Please note that if the MIDI file has more than one track, each track will be loaded on to a separate track and new tracks may be created if necessary.)  You may be able to download or purchase MIDI files that contain popular beats and rhythms.