Looks to me like:
is not written correctly. A post of what is displayed when:
is entered in terminal-mode would be helpful. Only one online repository must be enabled and the syntax of the line enabling the repository must be correct.
[rg@localhost ~]$ cat /etc/apt/sources.list
# Package repository URL's
# Signed repositories have a [<key>] where <key> is the name of the key
# as it appears in vendors.list. If you remove it, no digital signature check
# will be made.
rpm file:/media/disk/Repository/ pclinuxos/2010 kde4 nonfree
rpm file:/media/.22.0-16pcl/Repository/ pclinuxos/2010 main special
rpm file:/home/rg/Repository/ pclinuxos/2010 extras updates
# rpm http://pclosusers.com/pclosfiles/ pclinuxos/2010 main updates nonfree kde4 games
# rpm http://ftp.heanet.ie/pub/pclinuxos/apt/ pclinuxos/2010 main updates nonfree kde4 games
The last two lines above (my /etc/apt/sources.list) are for online repositories, which are disabled. Note the spaces in the lines of code. I am using three local repositories presently, explaining why my sources.list looks different from most.
An enabled repository will lack the # character at the beginning of the line.
If Synaptic is unable to detect a usable online repository, as in your case, it will also report problems downloading package lists.