The MySQL extension for PHP must be loaded correctly through the php.ini
Especially on Windows platforms, this can be rather troublesome as there are multiple locations where libmysql.dll file can be placed (PHP directory, MySQL directory, Windows system directories)
Adding a phpinfo file to your installation (a file with just the contents:
can help you track down whether this extension is correctly loaded.
Unfortunately the documentation is lacking in this area, but it is an Apache/MySQL problem and not specific to using XOOPS