\documentclass[oneside,12pt]{memoir}
\usepackage{xltxtra}
\usepackage{lastpage}
\usepackage[T1]{fontenc}
\makepagestyle{plain}
\makeoddfoot{plain}{}{}{Page \thepage\ of \pageref{LastPage}}
\makeevenfoot{plain}{}{}{Page \thepage\ of \pageref{LastPage}}
\begin{document}
\pagestyle{plain}
Page 1
\newpage
Page 2
\newpage
Page 3
\newpage
\end{document}