Download, unzip, and copy to your content/mpeg folder (on XP look in C:\Document and Settings\All Users\Application Data\ElectricSheep, or on Win7 C:\ProgramData\ElectricSheep, and on Mac ~/Library/Application Support/Electric Sheep, and on Linux in ~/.electricsheep). Note that on Mac that Library folder is in your home directory but is hidden, see this article for how to get to it.
If you want to install a bunch of packs (more than 2GB total) you should first increase the cache size under the settings control panel, or make it unlimited.
On Windows, that's in a hidden folder, you may have to go to "View" under the "Folder Options" menu and check the "Show Hidden Files and Folders" box. You can go there directly from configuration/settings window by clicking to the advanced tab, and then click "opening this folder", and put them in "mpeg".