Outlook 2010 by default has Direct Booking disabled. This is by design since Direct Booking is the old Exchange 2003 way of doing things and Exchange 2007 and 2010 has automatic processing of resource mailboxes.
In order to enable it in Outlook 2010 you will need to add the following Word in the registry for the organizers client:
Key: HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Options\Calendar
DWORD: EnableDirectBooking
Value: 1
reg add “HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Options\Calendar” /v “EnableDirectBooking” /t reg_dword /d 1