You ask either of them to point you towards the way that the other guy would say is correct.
Let's say that the correct way is to the left:
If you ask the lie teller, he'll say that the truth teller will point you to the right. (The truth teller would actually point you to the left but you're getting your information through a lie-teller, so the information gets reversed).
If you ask the truth teller, he'll truthfully tell you that the other guy would lie and tell you to go right.
So you'll end up with the WRONG direction in either case, so you go in the OPPOSITE direction to the answer you've just been given ;-)