In Debian, you first need to set up repositories, if you did not set up repositories, the only packages that appear in Synaptic are those that are on the CD. Debian repositories have something like 11000 packages or more, so you should be able to install the package you want through Synaptic and not from source. Here is some reading material to get to know how to set up repositories:
The sources list can contain all kinds of sources (download mirrors). I have Debian Etch 64bit, here is a copy of my sources.list:
The lines with a hash at the beginning (#) are commented out and not read by apt, if an entry was not commented out and invalid, apt would give an error during the "apt-get update" step that should be performed before every time you go to use synaptic, the first time you do "apt-get update" will take a little time to download repo data, but after that it only downloads changes and goes allot faster. In my apt sources.list are three sections that require one to download a gpg key to verify packages, they are directly under the title and are applied with the two commands that are commented out, for example:
For Multimedia you would copy and paste the gpg key commands into a terminal as root one at a time to get the key installed on your key ring:
gpg --keyserver hkp://wwwkeys.eu.pgp.net --recv-keys 1F41B907
gpg --armor --export 1F41B907 | apt-key add -
Once you have the key, apt will not complain about invalid package signatures when installing something. Also, in my sources list are two sources from university of Yorkton Saskatchewan because they are close and fast for me, you would need to set up mirrors closer to you. In the howto above (web link) is a link to all Debian mirrors to choose one close to you.