First u need Modern Warfare 2.
1. Load up Modern Warfare 2 and go the the multiplayer screen where you can see your nat-type which it should say is open.
2. Press eject on the PS3 so the game ends and put Black Ops in it.
Do not turn off or reset your ps3 after ejecting MW2.
3. Go now to Multiplayer --> Options Screen and press select, it should stay open and you can play with friends.
This works for me every time! Probaly MW2 opens these ports so that Black Ops can use them once you swap over.
Hope I helped you guys with this, give me some feedback please!