To unlock endless fever, you must complete ALL of the levels in the game TWICE. Your Music History and Puzzles should be complete and you should have beaten all of the Mini-games (You don't need to find all of the enemies to get it). After you have the above completed, go to the shop. It will be the last item for you to buy, but it doesn't cost anything. Endless Fever prevents your emotion meter from ever draining. If you want to turn it off, you need to go the the menu and go to extras to turn it off.