I believe that this is controlled by the Effect that your ability uses. Find your Effect and go to the Targets tab. Find the "Target Location Type" and change it to Unit. Then, go back to your Ability and go to the Ability tab. Scroll down until you find Target Filters. Add a new object and select the type of units you would like to be targetable. Anything you don't want to be targetable, either uncheck (in the case of the players) or change to excluded.