After you finish a level you will go to a bonus round. There you will have to find all the rockets and launch them on the top of the suntops and moontops. To get there bounce from star to star. If you don't launch all the rockets by when time is up, you will not get a password.
Note: you can find duck heads for extra lives at the menu when you turn your system on you will see a word that says: password. Select that and punch in one of passwords below: