Plugins are installed (and un-installed) using the Plugin Manager panel accessible from the JaxoDraw Options menu. First you need to download the plugin jar from the corresponding plugin's web page, then use the Plugin Manager to install the plugin.
Once a plugin is installed, JaxoDraw will automatically recognize it at start-up and the corresponding functionality will be available for the current session and subsequent sessions until the plugin is uninstalled.
Equivalently, you may just drop the plugin jar into the
JaxoDraw plugin folder
($HOME/.jaxodraw/$VERSION/plugins/).
All plugins and related information are locally stored in this directory.
To un-install a plugin, remove the plugin jar (and any corresponding
property files) from this folder.