Update 31/12/2012: Now this is a bit simpler, since bug 448414 has been fixed.
Before we start with the step-by-step procedure to get the mailman plug-in working, lets recall how to test a munin plug-in:
$ sudo munin-run mailman posts.value 24 members.value 21892
The output above is what you should get once the mailman plug-in is correctly set-up.
Now, lets start with the step-by-step guide. First, unmask munin, since we need version 2.0.9-r1 at least:
Install the plug-in:
$ cd /etc/munin/plugins $ ln -s /usr/libexec/munin/plugins/mailman
Configure the plugin:
/etc/munin/plugin-conf.d/mailman [mailman] user mailman group mailman
Restart the daemon:
# /etc/init.d/munin-node restart