I found a way to display MathJax in an EverNote note -- but the trick is to display it in WordPress. WordPress has plugins for both EverNote (plugin is called EverPress) and MathJax. Set the EverPress plugin to link to a shared notebook in your EverNote account (and whatever other settings you want for displaying notes). When you want to include some MathJax in a note, you need to double-escape the LaTeX commands -- so, for example, instead of "\(\int e^x\, dx=e^x+C\)" in your note, write it as "\\(\\int e^x\\, dx=e^x+C\\)". Then when the note displays in WordPress (with the MathJax plugin), the LaTeX will be correctly typeset in the WordPress page. Until someone finds a way to get the EverNote site to actually use MathJax, this looks like the best option available.