First join the Beta_Program and wait for Vuze to update itself with the latest software.
The plugin is available from the plugin installation wizard within Vuze - look for 'I2P Helper'.
From this version the plugin is available to install via the plugin installation wizard, see below.
Prior to version 5400 of Vuze you need to be running the latest Beta version of Vuze for the plugin to work - this is due to core changes that were required to support the plugin.
If I try to install it now, it runs into the old version and wont uninstall it for some reason.