Hey there!

With one of my characters i unchecked the activate box in the addon menu.

ESC -> Interface -> Addons -> Rotation Master -> Activate box

The Rotation Master entry is now missing in the addons menu in interface. Other characters still have this entry.

How can i reactivate it? Are there slash commands like /rm active or so?

There should be a slash command /rm toggle, which toggles it enabled/disabled. Have you tried that?