I found an old post saying that MathJax is enabled using the ∑ button in the Edit window. But this button doesn't seem to be there.

 

I'm pretty sure you can just use math bbcode tags, as in

[ math]\frac{-b \pm \sqrt{b^2 - 4ac}}{2a}[/ math]

without spaces produces

[math]\frac{-b \pm \sqrt{b^2 - 4ac}}{2a}[/math]

Unfortunately you have to reload the page after you post for MathJax to notice the tags in it...