See my example screenshot. This ^^^ toolbar does not show up on my site. I use MyBB 1.8. I've tried installed sceditor, yui editor, million different text editors but it never shows toolbar.
Js error or a bad theme version with MyBB version.
Solution: fix broken js or change a new theme.
Some plugins give this error too, but as i see it is a problem with your theme.
Jquery no conflict
and disable rocketloader if you are using cloudflare
installed sceditor plugin but changed compatibility to 1.8 and it worked