Let me ask you first; are you a user or an administrator in this context?
The Global Address List is being maintained on the mail server by your mail
admin. As a user, you cannot add any contacts to it.
So, if you are the admin; AD U&C stands for "Active Directory Users &
Computers" and you can at least find that back in Administrative Tools in
Control Panel on the Exchange server.
If you are a user, you can ask your mail admin to add that contact but
realize that it will show up for all users in the corporation. Private
contacts can be placed in your Contacts folder.