Yes, you have slightly the wrong idea about what synaptic is and how it works. Synaptic doesn't install .tar files, it installs .deb files in Ubuntu. You have to let synaptic download and install the files as it is configured to. You can change the locations, but there really isn't any reason to do so. Tar files are meant to be compiled most of thetime, and synaptic doesn't compile, it just installs.
If you want to have something that builds from tar files, I'd suggest looking at the emerge function in Gentoo, IIRC it will install apps from .tar files. Slackware may have an installer app in 10.1 as well, but I firget the name as I like to do the full compilation/installation myself.