You can loop any clip by dragging. Move the mouse to the left or right edge until the cursor turns into a left right re-size cursor .
Simply click on the left or right edge of the clip, hold the mouse down and drag left or right.
Each loop is represented by a triangle on the top and bottom of the sound.
The clip will loop based on the snap settings. (If you cannot change a clip to a specific length it is probably because snap is on and you need to turn it off.)
Alternatively, you can click on the "+1 Loop" button on the top left portion of a clip.