Citrix,
You would want to make sure the client you are using AD users and computers from is
1) In the same Domain as the DC you are trying to manage...or....
2) In a Domain that has an established trust relationship with the Domain that contains the DC you are trying to manage
If you have fufilled #2, make sure the account you are using to logon is a member of the Enterprise Admin group. Domain Admin would not do it in that case.
It wouldnt make sense for someone to be able to load up AD Users and Computers and then begin managing any domain they want. If that was the case, I could manage yours myself!