Because you are inside math mode, the single quote character has a different meaning. Since quotation marks proper are part of the text, a better way to do this is to use the amsmath
\text
command. Since you want to put an actual math expression inside the quotation marks you need to put just the quotes themselves in the \text{}
macro. If this is something you might do often, then turning it into a macro might be a good idea too.
\documentclass[xcolor=svgnames]{beamer}
\usecolortheme[named=Crimson]{structure}
\usetheme[height=7mm]{Rochester}
\usepackage{amsmath}
\begin{document}
\begin{frame}
\begin{align*}
\text{``}a^2 + b^2 = c^2\text{''}
\end{align*}
\end{frame}
\end{document}
"
(single character) instead of two'
. – Fran Jan 12 '14 at 20:41amsmath
package and use\text{``Formula''}
– Alan Munn Jan 12 '14 at 20:45\text{``} Formula \text{''}
. – Thomas Jan 12 '14 at 20:47