The short answer is you need to buy a new font. Out of the box, Office 2010 and 2013 have only Cambria with all of the math characters that are needed for the Equation Editor. You can verify this is also your circumstance by clicking expand link at the bottom right of the TOOLS box in the EQUATION EDITOR > DESIGN tab (see the screen shot to the right)
After some considerable effort, I have still not found a compatible font so I have posted this with Microsoft Partner Support and after some back and forth, this is what they said:
…Cambria Math is the only supported font that can be used in Equation Editor in Word 2013 for the time being. Anyway, the DEFAULT FONT is a drop down box and it will have alternates after some improvements are made by the developing team.
A slightly longer answer is the work around of forcing your equation to “NORMAL TEXT” (see the screen shot to the right). The tells Word that you you are not writing math, just normal text which will then allow you to change the font at will. The risk here is that you will lose characters because they don’t exist in your new font (say Arial).