Board Thread:Support Requests - Getting Technical/@comment-11700523-20160506014541/@comment-1757994-20160506031408

Go to Special:Preferences and make sure "Enable personal JavaScript" is enabled for yourself.

That enables your personal common.js, monobook.js, and wikia.js on every wiki, plus global.js on Community.

If you want to enable js for the entire wiki, you'll need to make sure ContentReview is enabled on the wiki. If it isn't, ask for it at Special:Contact/general on your wiki.