Go to:
- Administer->User Groups
- Click “Create User Groups”
- Select the “Roles” tab and check off “Admin”
- Select the “External Groups” tab
- Enter the name of your AD group
- Click submit
If you have entered all of your info correctly when you created your LDAP source, it should Just Work ™ if not, you’ll need to go back and work out what went wrong (Eg, wrong Groups Base DN most likely)