installing software from ubuntu software center using an internet proxy server

I am a first time user.I Installed ubuntu 11.10,and I am using a proxy server to connect to the internet.
It is connecting well, and I have given the proxy in firefox,as well as @ the system settings menu also
However, I am not able to install any software using ubuntu softaware resources,as no download is happening.
When I tired to use the terminal to install synaptic, the following command is shown

anoop@ubuntu:~$ sudo apt-get install synaptic
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package synaptic is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'synaptic' has no installation candidate

I wanted to install some statistical computing packages which is necessary for my research.
So,please answer this query.Any kind of help will be highly appreciated