In our new installation of 2.7, from the LiveDVD, MathJax was not working. I found that the problem was that in webwork2/htdocs, there were two folders: MathJax (which had content) and mathjax (which was empty). I deleted the latter and renamed the former with lowercase letters, and now MathJax seems to work.
So somewhere in the system some case sensitivity was preventing MathJax from working. I didn't personally install this server, so I don't so I can't say for sure if this is a local issue. I thought it would be worth noting in these forums.
If one does a WeBWorK install from the live DVD, MathJax is set up correctly in the directory .../mathjax. Whoever set up your system must have made changes probably trying to upgrade MathJax. The liveDVD documentation refers people to http://webwork.maa.org/wiki/Installation_Manual_for_2.7_on_Ubuntu_12.04#Checking_for_and_installing_hotfixes where one can find info on MathJax under the section: "Updating MathJax".