I have server 2008 with service pack 2. The security Tab of the builtin Admin is missing. What have I done to cause this?
You need to turn on Advanced Features in AD Users and Computers.
To do so, open ADUC, go to the view menu, click the Advanced Features option. The tab should now be visible.
This is obviously assuming you are looking in AD Users and Computers, and it is the Administrator user account you are looking at, and not the Administrators group.