For a detailed guide on how to install plugins, see here.
In summary, use the plugin command that ships with the IdP to install the plugin from either a local file pre-downloaded, or from a URL.
Installation
$ /opt/shibboleth-idp/bin/plugin.sh -i <plugin.tar.gz>
or
C:>\opt\shibboleth-idp\bin\plugin.bat -i <plugin.zip>
If installing from a local file, you need to ensure the GPG detached signature (e.g. the .asc file) is placed alongside the main plugin archive on disk.
Listing Installed Plugins
$ /opt/shibboleth-idp/bin/plugin.sh -l
or
C:>\opt\shibboleth-idp\bin\plugin.bat -l