Directions on how to get there: Go to Varrock square and walk south, across from Thessalia's Fine Clothes in Varrock, You should see a big building that has a Picture of a pint of beer, it is the Blue Moon Inn, Go in there.
In the Inn, go upstairs, and search around for a treasure chest. Use the key on the chest. After opening the chest, the key disappears. You can now continue with the Pirate's Treasurequest.