My advice is take it to a qualified vet. I can see the spot you speak of, but I cannot see how serious it is. It could be just a normal marking, I just can't really tell for sure. I cannot see any pus oozing from this spot, but if there is indeed pus, then you need to see a vet.
And I will back up what John said, you really can't trust pet shop employess to give solid advice in most cases, and thats not just for amphibians, it goes for reptiles, fish, inverts, birds, etc. Most don't have any real experience or training in the field of exotic animals. Of course there are exceptions to the rule, but you generally don't find these people at Petco or Petsmart.