Active Directory Migration Tool

When trying to use the Active Directory Migration Tool to migrate Windows NT 4 Server user accounts to my Windows 2000 Server.
Assuming that the NT4 domain is ‘NTDOMAIN’. The Windows 2000 domain is ‘win2k’.

However when i choose the option ‘Test the migration settings and migrate later’.
(Where the Source domain is ‘NTDOMAIN’. The Target domain is ‘win2k’.)
I get the error ‘You are not the administrator on the source domain.(domain=NTDOMAIN)’
I actually configure trusting on both servers.
I tried to add the administrator to the Domain Admin Groups on both servers but it seems that there are no options to do so.
What else do i need to do ?