4
In XOOPS 2.0+ - in the administration, every page has it at the bottom.
In XOOPS 2.2+ - Default installation has it in the footer of every page, or some themes has it there. But it's not that sure.
In both verions it's in file it's in file version.php which is in the /include directory (/include/version.php).
There are olso other ways, like adding some code into theme.html etc., but I guess those are rather complicated.