Recently I was working on an older Joomla website (v1.5.11) and received the warning message "Parameter 1 to modMainMenuHelper::buildXML() expected to be a reference". After some research, I discovered that older versions of Joomla and newer versions of PHP (specifically v5.3.0) do not mix well.
The best solution for this problem is to update Joomla to the newest version (1.5.15). After I had updated Joomla, the warning message disappeared. This thread on the Joomla forums documents a user having the same problem with a Joomla v1.5.12 website and fixing the problem by updating to Joomla 1.5.15.