Hi, I am trying to understand what you mean here. Are you trying to play the main game as the Joker, or the challenge rooms?
I only ask as there are about ten skins for the 'game', which are all batman (more or less), where as the 'challenge rooms' allow you to play as other batman universe characters.
The challenge Rooms are a separate campaign, which are arena type survive as long as you can sort of things. Accessed via the main menu. The joker ones are highlighted at the top of this sub menu.
Hope this helps.
Hush little baby don't say a word, mom as gonna blow up the whole da#n world.