This could be one of several issues. Not to offend - But are you sure you opened the seacock? If not, you would build up pressure, overheat the pump and possibly trip the breaker.
Assuming that isn't the issue - There should only be one pump: Between the holding tank and the seacock - It "Sucks" the contents of the tank and then pushes it out of the seacock. If you installed the breaker yourself try bypassing it and see if the pump comes on. As far as I know there is no manual way to activate the pump so You'll have to diagnose your way back to the pump and if you find nothing you may have a bad macerator (Pump).
Do you have an electric head? I can't remember if the head on a 265 is electric or manual. If it is electric make sure the "Head" is turned on also. I believe the 265 is a saltwater head. Make sure the saltwater intake is shut off and only the discharge is open. Good luck - That's a "crappy" problem to have.
