Google earth is proprietary software, so it's almost assuredly not in synaptic.
In Debian/Ubuntu (which also use Synaptic, so I'd guess you're in the same boat), the repository entries for google earth are simply the libraries that google earth needs to link against during runtime. After you install those libs, you still have to go to http://earth.google.com/
, download & run the installer to get the actual google earth program.
Hint: run the installer as root or as a sudoer to install for all users on your machine, otherwise, you'll install it only for yourself (if that matters).
once it's installed, it will show up in your Applications menu under "Inernet"