I think I may have a fix for this issue. I had this problem on two different OS X machines both running Lion and Apple Configurator. The issue appears to be related to something broken in the key chain. Exactly what, I do not know and may vary from system to system. I repaired my key chain, closed and re-opened Apple Configurator and was good to go.
Follow these steps:
1 .Go to Applications -> Utilities -> Key Chain Access
2. From the Keychain Access menubar item, select "Keychain First Aid."
3. Enter your credentials and click "Verify." If any issues are detected, which I suspect you will have at least one, proceed to the next step. If not, maybe this isn't your issue afterall 🙂
4. Enter your credentials again and select "Repair."
5. Open Apple Configurator and retry!
I hope this fixes your issue, it worked twice for me.