We have used WDW passes at DL a few times. The procedure is odd to me. Both times we were to Guest Relations first, because this seemed logical. But NO, they want you to approach the entrance as usual, and tell the ticket turnstile CM there that you have a WDW ticket. They will have you sort of step aside, and they get on the radio, and someone shows up to assist. This second CM takes your pass, and apparently goes to call WDW on the phone to verify it is valid, and then WDW removes a day from the pass. Then this CM comes back with your WDW pass, and also gives you a one day hopper, good for that same day at DL/California Adventure.
It seems strange, but has always worked fine. We buy 10 day no expires for WDW, so if we need an extra day for DL here and there, it works well to take one day from the WDW pass.....