In Guitar Tracks Pro 4, I'm trying to create a simple MIDI drum measure in the Piano Roll View, then repeat it 100 times in the track.  The instructions on pg 60 of the Getting Started guide say "To create additional repetitions of the loop, point the mouse pointer to the clip's right edge, then drag the clip edge to the desired duration.".  When I do this, the loop doesn't repeat, it just extends it out to where I let go of the mouse button with no notes in the area I just created.  

What is the right way to repeat a MIDI clip?

The clip you have created is not a loop yet.
Go to the track view. Note the corners of the clip are square. 
You may want to turn on Snap to Grid to be sure the selected clip lines up with your measures.
Right click on the clip, and click "Enable Groove Clip Looping". You now have a loop, and as a visual cue, the corners are clipped.
Now you can drag out the right edge of the loop per the instructions.

worked perfectly, exactly what I needed, plus fast response!  Do you work for Cakewalk?

I must have missed that instruction in the book AND help file.  Thank you!

No, I don't work for Cakewalk.
I'm just one of many users you will see here, helping where we can.
You're welcome, glad to help.
Btw, welcome to the forum.