For Domain joins, initially leave the OU field blank as there's a bug in the hostnamechanger.dll which causes the domain join to fail IF you specify an OU, AND IF the PC already has an object in AD. I posted a fix for this on the FOG site (the error you get is 2224).

If you do eventually specify an OU it needs to be in LDAP format, e.g.

OU=Workstations,DC=mydomain,DC=co,DC=uk

Otherwise your settings should be hopefully be OK now. Just ensure the user account you are using has sufficient rights to join PCs to the Domain.

Good luck!