How i can enable it for specific menu…

Yes, assuming your theme properly implements wp_nav_menu with the theme_location parameter, you can select which theme location to apply the RSM to in the Control Panel.

Hi this is old, but I want to apply this only to a specific sidebar menu. Is there anyway to do this?

If by ‘sidebar menu’ you mean a custom menu widget, then no. Only menus that use the theme_location parameter can be targeted.

My problem is the exact opposite. RSM is being applied to a custom menu in a widget and I don’t want that. I see these instructions:

Selectively Activate Theme Locations
Disable the above and activate only the theme locations you want. These theme locations correspond to the Theme Locations Meta Box in Appearance > Menus

But I don’t see any place to apply this. How do I “activate only the theme locations” I want?

Thanks

If you don’t have any theme locations listed below that, then your theme hasn’t registered any theme locations for you to activate it on.