problem connecting to at&t network with sony ericsson w880i?

I just bought an unlocked sony ericsson w880i and it works great. only problem is that it cannot connect to at&t. Currently I am trying to manually connect to the network and it has been searching and has come up with no results in the past hour. Any ideas? I called at&t as well as sony ericsson and they both could not help me out. Has this problem occurred to anyone and if they have how do you fix it? THANKS!!