Although I have not used the slide creation ability of Latex, have you
tried using:
\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyfoot[CE,CO]{\thepage}
Alternatively, you could change \fancyfoot to \fancyhead and use those
options to put the page number in the ceter of the od and even header.
NB CE = center even CO = center odd
Hope this helps
James