Do you have the "Allow Manual Backups" enabled? If this does not work, what you may have to do is install the server client software portion on the machine in question and make it a client. Then you can specify the user you desire to perform the restores in the Remote Administration area like so: "username@machinename" or "user@domain.name"
Good luck. Watch your lisence usage here, remember this last mentioned method will take up a client.
CF