Without the details of the error it is hard to know what is wrong. I suggest searching the internet on the error message to see if anything comes up. Since I don't know much about AD, that is basically what I would do if you posted the error message here.
If nothing else, you can update the aspnet_UsersInRoles table directly to add the user to the role. Off the top of my head, though, I am not sure what you would use for the UserId column.
Roger Martin
Creator and Lead Developer of Gallery Server Pro