The customs office is located on Motu Uta. It is a large 2 story white building with a large "Douane" sign out front. You can take your dinghy over there.
Enter the front door and walk down the hall way almost to the end on the right you will notice a plaque "Yachting".
Present your old duty free certificate if renewing or your green enterance paper given to you at the gendarme at first port of entry.