Why are the HideElements menus for hiding elements globally or on a space level not showing?
Global and space-wide hides were introduced with HideElements 3.0. This version is currently only compatible with Confluence 7.
If you are on Confluence 6 you probably have HideElements 2.x installed. This version only comes with the HideElements macro.
You can confirm which version of HideElements you have installed if you go to "General Configuration > Manage Apps" and click on the HideElements entry.