Canons in Castrum

I can't figure out how to fire cannons in Castrum. Asking in pugs only results in people ignoring me or calling me noob.

So I get the ammo. Let the loader load it to the cannon. Click on the cannon. A red shooting icon appears in my skillbar. I make sure the huge yellow cycle is pointed at Livia. And then what? When I try to use that skill nothing happens. I'm playing on PS3, if that makes any difference.

If you've hidden your pet bar, that is where the command is to fire it. You'll click(select) it, and then place a targeting reticle on the ground.. and click/whatever you do on ps3 (I know I originally had the pet bar hidden and had a problem with this)

i play ps3. if you have the circle under her, just press x.