Looping Virtual Instrument Clips
You can make a Virtual Instrument clip loop.  In the Piano Roll Editor, you can adjust the start and end loop points.  To move a loop point, simply click on the word Start or End and drag it.

For example, we'd like to make a loop from measure 2 to measure 3.  Here is what we started with:

In this example, we drag the End loop point to measure 3 and then drag the Start loop point to measure 2 like this:

Once you are satisfied with the loop, you can extend the clip in the session area to loop it as many times as you'd like.  See Looping Sounds.