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A short how to do to Install Munin-Plugins which works fine to me..

the Plugins are located in the plugins directory (Ubuntu 12.04 lts)
"ls - a" will show you what you have 

cd /usr/share/munin/plugins/
ls -a

first find your needed plugin on github  --> contrib --> plugins

when you find it select it and click on raw file - you should see the coded file copy the right url

use the wget command to download the file into your directory (wget and paste the right url)

sudo wget https://raw.github.com/munin-monitoring/contrib/master/plugins/mail/xxxxxx

 xxxx is your new file

sudo chmod 755 /usr/share/munin/plugins/xxxxxx

if needed, customize the file to your needs

Now create a link to the place where the acitv Munin Plugins are

sudo ln -s /usr/share/munin/plugins/xxxxx /etc/munin/plugins/xxxxx

To verify, run

sudo cd /etc/munin/plugins/
munin-node-configure

at last but not least

/etc/init.d/munin-node restart

and now wait 5 Minutes to watch the graphs