Plugins are loaded by either using the loadplugin script command or alternatively by putting a <plugin> element into the XML file. The latter is the recommended method, because it is simpler and allows you to attach event scripts to objects that the plugin registers. Here's a simple example of loading a plugin using a <plugin> element:
... <page ...> <plugin module="myplugin"/> ... </page> ...
Note that the lifetime of your plugin is dependent upon whether you put the <plugin> tag inside a page or in the <system> section. If it is within a <page>, it will be in memory only while the page is being displayed. If you put it in the <system> section (or loaded by using the loadplugin script command) it will be loaded for as long as eboxy is running, unless the plugin requests to be unloaded.
eboxy will look for plugins in LIBDIR/eboxy only (where LIBDIR is picked up at compile time, usually /usr/local/lib). Also, the .so extension is always omitted. So in the above example, when the page is loaded eboxy will attempt to load the plugin file /usr/local/lib/eboxy/myplugin.so into memory (actually, some loading and pre-initialisation is done when the XML file is read in, and the plugin is fully initialised when the page is loaded).