Using AT&T with Tmobile Sidekick iD?

I have followed the directions exactly. Put in SIM go to settings-SIM center-Change SIM pin- changed it to somethinq simple like 1111 and i didnt get any bard. I can do the network thingand click AT&T but it will only let me call not text. It still says SIM? and its blinking in the top right area. Any other ways to do it?