Man md5 displays dgst man page
On my ML installation, man md5 does indeed show the MD5(1) man page.

It's possible some local software installation or configuration is causing the display of a man page for dgst instead, though (which appears to be part of OpenSSL.)

A few things to check:

Do you have the MANPATH environment variable set in your shell?  It could be looking in an alternative directory for manpages before hitting the system manpages.  MANPATH does not appear to be set by default.

Are there alternative MANPATH directives in /private/etc/man.conf?  My system has

MANPATH /usr/share/man
MANPATH /usr/local/share/man
MANPATH /usr/X11/man


Has /usr/share/man/man1/md5.1 been replaced by a symlink to another man page that could be producing the dgst content?  Try

$ ls -l /usr/share/man/man1/md5.1
-r--r--r--  1 root  wheel  12332 Nov 16  2012 /usr/share/man/man1/man.1


If /usr/share/man/man1/md5.1 has not been changed, then you can read its content specifically this way:

$ man /usr/share/man/man1/md5.1