Did you get the quest to find the supply depot? Get the hint from Tingalf that the depot was nearby, under his feet? When you first go down the sewer from the Ars Magicana entrance and take a few steps east, there should be a short cut scene where Pedro asks you what you are doing there. Then, after a short conversation, he orders everyone to attack you. If you found him without getting the quest by going down a different sewer entrance, that could be the problem. If so, ignore him until you get the quest. If not it might be a problem with the cut scene.
