From Windows:

If you have downloaded the .zip file, you can extract the files from that archive directly using Windows XP's zipfile support. If you downloaded the .tar.gz file or if you are using a Windows version that does not have built-in zipfile support, then you will have to download an appropriate tool for unpacking, e.g. WinZip (http://www.winzip.com/).

From UNIX:

If you have access to a UNIX shell, you can unpack the .tar.gz archive using the following command:

shell> tar xvzf phorum-x.y.z.tar.gz

Or if the tar program on your system does not supprt the "z" flag:

shell> gunzip phorum-x-y-z.tar.gz
shell> tar xvf phorum-x.y.z.tar