If you are are to see the dungeon in the duty finder, then someone else in your party has not made it available yet. Perhaps they are making the same mistake my friend always makes.. he gets the quest that says go to such and such dungeon, but he forgets to actually go there and activate it. Its also possible that you or someone else has their chocobo out.. I have no idea if that counts, but I do know that chocobos count as party members.