You must ask barkeeper about Reynard and his friend, next, you ask them about stealing from customers, they will give you a lockpick and a silver crest, then you must find a ancient map (i forgot where did you get it) then a tomb should be at the top right corner of the map. Enter it, then place the crest at the center of the rock in the triangle-looking place to release Erza. Do what Erza says and then you will able to get to the dungeon