Formal:
* I have no knowledge of that.
* I am unfamiliar with that.
* I am not aware of that.
* That is beyond my knowledge.
Informal:
* I'm clueless.
* I haven't a clue.
* Beats me.
* No clue.
* I'm drawing a blank.
* I'm stumped.
* Not a clue.
Slang:
* No clue, man.
* I'm clueless, dude.
* I'm lost.
* I have no idea, bro.
The best expression to use will depend on the context and your relationship with the person you are talking to.