Ubuntu 9.04 or older
First you have to add the PPA to your packages sources. For this, go to System -> Administration -> Package Sources. Then go to the Third Party Software tab and add the software sources matching your Ubuntu version.
Ubuntu 8.04 (Hardy):
deb http://ppa.launchpad.net/jan-hoffmann/ppa/ubuntu hardy main deb-src http://ppa.launchpad.net/jan-hoffmann/ppa/ubuntu hardy main
Ubuntu 9.04 (Jaunty):
deb http://ppa.launchpad.net/jan-hoffmann/ppa/ubuntu jaunty main deb-src http://ppa.launchpad.net/jan-hoffmann/ppa/ubuntu jaunty main
Then close the Package Sources application and click on reload if you're asked to reload the package cache.
You have to add the key which is used to sign the packages manually. Open up a Terminal (System -> Accessories) and type in the following command. You will be asked for your password.
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 9C245DE2
Now you can install ACPI StatusIcon by opening Synaptic from System -> Administration, searching for acpi-statusicon, marking it for installation and applying the changes.