I recently renewed my w101 membership after 4 or 5 years of nothing. I used to have Mooshu unlocked, but now I don't for whatever reason. I have "The Keys to Success" (I think that's what it was called) quest on my ice wizard, so I know I got to Big Ben, but I don't have any Big Ben quests and I can't enter the dungeon. I looked on my lvl 63 fire wizard and that one could enter it, but not my ice wizard. I really need Mooshu for the iceblade quest. Help!
So, the Keys to Success quest is a side quest in Big Ben. It's a key given to you to open wooden, silver and gold boxes in the tower. It's not necessary to open these boxes in order to defeat Meowiarty. It's optional. So it could be you went through Big Ben already and just didn't open these boxes. If that's the case, you should be able to teleport back in and do Big Ben again in order to unlock the boxes if you want to complete the quest, but its really not necessary to in order to get higher level quests.
