Hi Andrew,
If you did a custom install you may have overlooked the Equation
Editor feature and not checked it in the install options.
No big deal, however. You can run the installer and do a custom install
and choose just Equation Editor (click the disclosure triangles in the
custom install to reveal the individual items available to be
installed). After you do this you will have to re-install all updates
for Office.
Another option is to visit the MathType web site and download the free
trial of MathType. Doing this installs a free font that has lots of nice
symbols in it. After the 30 day free trial ends MathType reverts to
Equation Editor (unless you like the full version, which of course
MathType is hoping for so that you buy it). After the free trial periond
you get to keep and continue using the font. If you install MathType you
won't have to go through another round of Office updates.
-Jim