Board Thread:Support Requests - Designing Your Wiki/@comment-4146635-20130624223048/@comment-1863391-20130626161957

You can not add elements to the Wikia sidebar globally. You can write a script that adds the element and then encourage your users to add that script to their personal js pages. If Wikia finds out that you are modifying the sidebar globally, such that the changes are evident to anons and people who haven't called the script manually, they will remove the script.

Edit - also, in my experience, the MDN is a great resource.

Edit again - now that I see that an old-timer like TK is suggesting that you contact staff, I'm not as confident in my original assertion. You might just want to do what he says ;)