Direct booking is disabled by default in Outlook 2010. To enable set this registry value :

Windows Registry Editor Version 5.00   [HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Options\Calendar] “EnableDirectBooking”=dword:00000001

Alternatly you can download ADM file from Microsoft and deploy.

http://support.microsoft.com/kb/982774