Mac OS X
For Mac OS X 10.3 (Panther) or 10.4 (Tiger) download the PKG file; uncompress the package (if it's not uncompressed automatically). The following instructions refer to this official PKG file. As an alternative there's also an unofficial front-end with a GUI, MacFreePOPs, that includes a pre-compiled executable for Mac OS X Jaguar, Panther and Tiger; for instructions and support you may refer to its own website.
Double-click on the .pkg file you got and follow the instructions. Be sure to read the ReadMe you're shown as it might include last-minute information and instructions on how to run FreePOPs. You will also be asked to accept the GNU/GPL License Agreement then to choose which directory to install to. Authentication as a user with administrator privileges will be needed.