STACK currently relies on MathJAX to display mathematics.  In particular the LaTeX mathematics environment.  There are two ways to write displayed equations.  We strongly recommend only using the forms with backslashes, \ [ and \ (, and not using dollars.  This page checks is mathematics is being displayed.

$\sum_{n=1}^\infty \frac{1}{n^2} = \frac{\pi^2}{6}.$

Inline mathematics: $\sum_{n=1}^\infty \frac{1}{n^2} = \frac{\pi^2}{6}$. 

If these examples do not display correctly please confirm your installation is complete.  You can do this by logging in as Admin and navigating to 

Site administration -> Plugins -> Question types -> STACK

Then choose the healthcheck script, which helps you verify that all aspects of Stack are working properly. There are also a number of other links which are useful for testing and for understanding how STACK interprets input and compares expressions.