Bernier -
First of all, PowerTracks only ships with a small number of RealDrums (3 sets if I recall correctly). You can always purchase additional sets to add to your capabilities. That being said...
There are two ways to get RealDrums on your track.
1. Highlight the area of the track for which you wish to generate drums. Then right-click on the track and select "Generate, Select and Generate RealDrums". You will get a screen where you can select from among your installed RealDrums (see figures 1, 2, and 3 below).
or 2. From the top line menu, select "Edit, Audio, Fill Track with RealDrums" (see figure 4 below). This will bring up the same screen as the first method.