First you have to hit O to open the rockets. Once you've done that, the rockets are mannable as the 7th gunner position. You can control the elevation with your stick at this point. I think that's how they work. Been awhile since I've used them.
I have no clue if they're effective or not. We've never gotten close enough to land targets due to ack, and usually the other PTs are opening up on you from about 1000yds away with their guns, so it's hard to use rockets against them. Once I scared the heck out of a plane straffing me with rockets.
I can't remember if I hit him or not, but I know he got me. Might have ended up being a mutual death.