First make a image in psp or what ever you use with what you want your credits to say.
Load that as a frame in your game. go to the frame properties and check the Enable the go to page timer .
Set interval to 0.1
Set the Destination frame to the start of the game or to a sec frame with more credits to .
Set the Transition to Slde Down .
Set the Duration to ( I use 8 but you try what you think is right) 8
There you go the credits will scroll. .