By default, the man program will look for man pages in /usr/share/man, /usr/local/man, /usr/X11R6/man, and possibly /opt/man. Sometimes, you may find that you need to add an additional item to this search path. If so, simply edit /etc/man.conf in a text editor and add a line that looks like this:

MANPATH /opt/man

From that point forward, any man pages in the /opt/man/man* directories will be found. Remember that you'll need to rerun makewhatis to add these new man pages to the whatis database.