In addictive drums click on beats, this will bring up all the sample beats in addictive drums 2, you can audition them in ad2 and if you want drag them from the Addictive drums GUI onto the midi track in your x3 GUI.

I just did this last night - here are the steps:
In the GUI for AD go to the BEATS tab and find a beat you like (I grabbed a simple 4/4 with open High Hat).
Drag it to the midi track feeding AD (or AD's simple instrument track if you have it set up that way) and place it at the starting measure of your choice (I put mine at measure 3).
Click on the clip to select it.
Use CTRL-L to turn it into a groove clip loop (corners become beveled).
Drag the right edge of the clip out as many measures as you wish.