Ubuntu 9.10
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 below:
deb http://ppa.launchpad.net/jan-hoffmann/ppa/ubuntu karmic main deb-src http://ppa.launchpad.net/jan-hoffmann/ppa/ubuntu karmic main
Then close the Package Sources application and click on reload if you're asked to reload the package cache.
Now you can install ACPI StatusIcon by opening Synaptic from System -> Administration, searching for acpi-statusicon, marking it for installation and applying the changes.