I did a search and study a few topics but couldn't find my answer.
I just disable Add Site activity from appearing on the outline by highlighting it and click SAVE (under Modules in ACP).
Now I want to activate it, make it re-appear on Outline again but I can't find a way to do that. (I don't really want to make it re-appear but I want to use the method to apply to other modules that I don't see in the Outline.)