xman is the standard X11 manual page (man page) reader program, and it comes with Apple's X11. You can run xman from Terminal, but it won't display man pages correctly. And if you add xman to the X11 Applications menu, nothing happens when you select it.
Before xman will work, there are a couple of environment variables that need to be set up. To add the necessary entries to your login shell, you need to edit the ~/.MacOSX -> environment.plist file, as described on this page at the Apple Developer website. The required entries are as follows:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030626224905185