In Lion to enable the root user you would:
- From the Apple menu choose System Preferences....
- From the View menu choose Users & Groups.
- Click the lock and authenticate as an administrator account.
- Click Login Options....
- Click the "Edit..." or "Join..." button at the bottom right.
- Click the "Open Directory Utility..." button.
- Click the lock in the Directory Utility window.
- Enter an administrator account name and password, then click OK.
- Choose Enable Root User from the Edit menu.
- Enter the root password you wish to use in both the Password and Verify fields, then click OK.
- Mac OS X v10.6.x
Not sure if this works for mountain lion as i havent tried it yet but that may get you there.
The only thing that worked for me was to put my old hdd back in, use it to create a bootable usb, use the usb to boot into recovery mode on the new hdd. re-format the new hdd and reinstall osx mountain lion.
Not sure how to mark this as a solved issue.
Used old hdd to create a bootable usb of max os x mountain lion
used the bootable usb to boot into recovery mode on new hdd
re formatted and reinstalled mountain lion
since I never had admin rights I could never install anything so I didn't have any info or files on the drive that I could lose.