In order to use the plugin, you need a PUMA useraccount.
Just follow these simple steps to experience the synergies of JabRef and PUMA:
Download JabRef
To use the PUMA plugin, you need an older version of JabRef. You can find one on this page: http://www.oldfoss.com/JabRef.html
Choose a version that is older than 2.12, for example 2.10:
Download the suitable file for your operating system and execute it. JabRef will then be installed.
Download the PUMA plugin
You can download the plugin here: https://gforge.cs.uni-kassel.de/frs/download.php/76/jabref-bibsonomy-plugin-2.5.2-bin.jar
Move the plugin jar file
Put the plugin jar file into a subfolder called Plugins in your JabRef installation directory.
Start JabRef
Start JabRef and you can start working with the Plugin!