Uh oh, you just added a user via Active Directory Users and Computers rather than SBS 2011 Dashboard. You've probably tried editing the ADSI properties without luck.
The essentials version of SBS allows home editions of Windows to be added so uses some new tricks, so the old way no longer works.
Heres how to add an AD user to SBS 2011 Essentials.
Browse to C:\Program Files\Windows Server\Bin
Double-click WssPowerShell.exe
At the prompt type import-wssuser and press enter, you'll be asked for the username, enter that and your user will become an SBS user.
