Synaptic is available in the so-called universe repository. If you cannot find that package, it means that you do not have universe enabled. To enable it, follow these steps:
GUI
Open Software Sources and select Community-maintained free and open-source software (universe)

After doing so, you will be asked to reload the software sources and you will be ready to install your package.
Terminal
Open the file /etc/apt/sources.list:
sensible-editor /etc/apt/sources.list
And add the following three lines:
deb http://archive.ubuntu.com/ubuntu/ quantal universe
deb http://archive.ubuntu.com/ubuntu/ quantal-updates universe
deb http://security.ubuntu.com/ubuntu/ quantal-security universe
Note: I'm assuming that you are using Quantal (12.10). If this is not the case, then use lsb_release -c to find the codename of your Ubuntu release and replace quantal with that codename.
Then run sudo apt-get update and install Synaptic.
and try updating the repo by running
– Srinivas Gowda May 09 '12 at 16:47sudo apt-get updateuname -soutput – swift Jul 11 '12 at 16:16uname -sis going to help here, should probably runlsb_release -rinstead to find the version of Ubuntu – tgm4883 Jul 11 '12 at 16:25synaptic is already the newest version., not the error he is receiving. – tgm4883 Jul 11 '12 at 16:28