Wow.... that's weird.
I hate to sound demeaning but you are using 7.5 Reloaded correct?
I ask this because from memorey with 7.5 (Windows version) It was achieved via a command line.
Did you install Plesk yourself? If not I would ask whoever did if they have any idea as to why this feature is not visible.
Sorry I have not been of much help to you.
Regards,
Rob
P.S. I just went through the manual and nowhere does it even mention this feature. VERY strange!!