When you buy the pass at CostCo you will get a voucher in the actual package that you buy in the store.
On the front of the voucher is a website address that you have to go to to register your voucher with Universals ticketing system. When you register it will ask for your name, DOB, address, and voucher barcode. Once registered, take your voucher directly to the main entrance turnstiles. Once your voucher is scanned, the turnstile will print out your season pass.
In the past, you had to take your voucher to the Annual Pass center to register, now its all done online.