From b2evolution manual
This manual page is outdated. NEW PAGE: Plugins in b2evolution v5+.
 What are plugins?
This way, the application can easily get extended without modifying its PHP files directly.
 Installing a plugin
To install a plugin in a 1.x Release of b2evolution, you simply have to put either the downloaded plugin file or folder (depending on what was given to you) into the
plugins/ folder. After that, you have to go to the App Settings tab for b2evo 1.9, or Settings tab for b2evo 1.8. Once there, you can install it from the list of "Available Plugins".
Some plugins might not get enabled right away, but need further configuration. You should receive a note about this then.
 Where to get plugins
We have a repository at http://plugins.b2evolution.net/, where you can browse plugins by category and version.
You can also view all the plugin information here on the wiki by viewing the Plugins Listing or by viewing the Category. Especially for the 2.x branche of B2evo there is a List of plugins that are known to work in B2evo version 2.2.0
 Creating a plugin