Copy/paste the following to the file /etc/apt/sources.list (as root)
deb http://ftp.au.debian.org/debian/ lenny main contrib non-free
deb-src http://ftp.au.debian.org/debian/ lenny main contrib non-free
deb http://security.debian.org/ lenny/updates main contrib
deb-src http://security.debian.org/ lenny/updates main contrib
deb ftp://ftp.debian-multimedia.org lenny main
Save the file, close the editor. Next, issue these commands, still as root:
apt-get update && apt-get dist-upgrade && apt-get install synaptic
The && dictates that each command is only executed if the previous one has finished successfully. The sequence: updating the package cache, completely upgrading your system to the latest versions of the installed software and install Synaptic. Let the machine finish.
It is wise to have a wired connection to the internet, at least until you've managed to get wireless working...