I wasn't thinking clearly and tried my AT&T phone account. I'll try the iPad AT&T 3G sign-in on the way back in the airport.
Note. At the hotel I must have had 3G on. I got on AT&T wifi right away. This morning I tried and got hotel Wayport for charge. But I went back to preferences and turned on 3G and I got free wifi right away. Maybe the AT&T 3G carries the credentials so that the partner wifi hotspots recognize the iPad?