PHP 5 should come with dom enabled by default. Which you can check by inspecting the php info output.
dom
DOM/XML enabled
DOM/XML API Version 20031129
libxml Version 2.7.8
HTML Support enabled
XPath Support enabled
XPointer Support enabled
Schema Support enabled
RelaxNG Support enabled
If you are using PHP 5 and for some reason dom is not available. Check for a package in your repositories called php5-xml