In order to get the Final Ending (+ Credits) you need to unlock all of the relax rooms.
Relax Rooms
To unlock the Relax Rooms you need to get line combos on TEN FLOORS mode. At the end of the TEN FLOORS game you will be shown a video from a Relax Room, the color of which will either be the highest color of line bonuses you acheived, or the next highest if you already have that Relax Room. There are nine Relax Rooms.