Download cfg_map.tar into the Nagios htdocs directory.
It already contains index.php, main.php, and side.php, etc.

cd to the htdocs directory.

Execute this in a shell / terminal.
    tar -xvf cfg_map.tar

Edit side.php and paste in the line below the line with "Event Log":
	<li><a href="<?php echo $cfg["cgi_base_url"];?>/cfg_map_object.cgi" target="<?php echo $link_target;?>">cfg map Objects</a></li>
You can paste it in any section you like, but that seems the most appropriate to me.

If you get error messages about "opendir" permissions, you may have to find the directory it references and:
	chmod -R 755 DIRECTORY_REFERNCED
Substitue your directory, of course.
If you don't understand the security implications of "chmod" please research that.
Honestly, if you don't understand those implications, stop using Nagios immediately.