Use the new grafana-cli tool to install piechart-panel from the commandline:
grafana-cli plugins install grafana-piechart-panel
The plugin will be installed into your grafana plugins directory; the default is /var/lib/grafana/plugins if you installed the grafana package.
More instructions on the cli tool can be found here.
You need the lastest grafana build for Grafana 3.0 to enable plugin support. You can get it here : http://grafana.org/download/builds.html
Alternative installation method
It is also possible to clone this repo directly into your plugins directory.
Afterwards restart grafana-server and the plugin should be automatically detected and used.
git clone https://github.com/grafana/piechart-panel.git sudo service grafana-server restart
Clone into a directory of your choice
If the plugin is cloned to a directory that is not the default plugins directory then you need to edit your grafana.ini config file (Default location is at /etc/grafana/grafana.ini) and add this:
[plugin.piechart] path = /home/your/clone/dir/piechart-panel
Note that if you clone it into the grafana plugins directory you do not need to add the above config option. That is only if you want to place the plugin in a directory outside the standard plugins directory. Be aware that grafana-server needs read access to the directory.