While in the game press “+” after that:
[F6] – rockets for player 1
[F7] – rockets for player 2
[F8] – lives for player 1
[F9] – lives for player 2
In some vesions the cheats are the next:
Press F9 and F10 together to enable cheats.
Now these keys will be enabled:
[F6] – Skip level
[F7] – More missiles
[F8] – More lives
To disable cheats press [F9] and [F10] together again.