You need to change the ability "Animate Dead" to your ability, "Signal".
Now as conan told me (and I've done it and made it work twice), you have to make a "Dummy ability" so that it doesn't do anything "special" like grant extra speed, extra armor, or something to your tower. You want to make it castable, and you want it to not click on anything. Anyway, go to object editor, find the ability called "Channel". Now, copy and paste it and look into the "stats" for said ability. Name it "Signal" where it says "Channel" and change Data - Options (make it Visible) along with Data - Target Type (to Instant (No Target) if it isn't already).
Now you're all set--but you may want to make the ability a "unit ability" if your tower isn't a Hero. Simply turn Hero Ability and Item Ability to false.