After hours and hours I finally got it to work. I thought I would chime in with what worked for me. I read through endless posts and tried many other peoples suggestions but many failed. This may or may not work for you.
I have a GT Ultra Express Card, Unlocked from NewEgg, AT&T Sim with FamilyPlan Data, OSX10.6.2.
I tried using the GlobeTrotter Software from
http://support.option.com/att/ This DID NOT WORK FOR ME.
I used the GT software from
http://www.option.com/en/support/software-download/, I used my serial as my IMEI didnt work. I installed with the card out, after install I inserted the card and started the GT software. It worked.
I dont know why, personally. I swear I did these steps before. The only thing different was the software package from the option download section.
Excuse my poor organization of thought, its been about 4 hours of trying. BUT, I have the airport off and am typing this message on 3G!