you need no gui to download the the compresses file on your server.
copy the download link for the version of ispconfig you prefer from http://www.ispconfig.org/downloads.htm
Then connect via iTerm (or Mac OS X's std. Terminal) via ssh to your server. Log in, type "wget " (without ") and then either hit the apple-key + V or do a right mouseclick and selcet insert (or just type the url by hand behind wget).
Wget will fetch the file for you with that you can perform the installation.