Use the backspace key to toggle between "secondary" fire items: rockets, bombs, drop / external fuel tanks, guns. Then proceed to use "b" or whatever button on your stick mapped to same.
As for the offline opponents, kill the green guys. Offline is not like the online world. By default the 4 P51's cirlcing your base are killable, but takes a lot more damage to kill them versus the online arenas. For practicing jabo, etc. go into the setup button, then arena settings and change the "protect objects" to off. That will allow you to destroy ack, fuel, etc.