Hit the Sigma/Sum button, ∑, then enter the LaTeX markup of a math expression.

For example, try entering this:

$$x = {-b \pm \sqrt{b^2-4ac} \over 2a}.$$

or this:

$f(x) = \int_{-\infty}^\infty \hat f(\xi)\,e^{2 \pi i \xi x} \,d\xi$

But make sure you select a good spot for it down here somewhere, $d_{i}$ as it will insert inline.