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.


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!