what you did was entirely correct.
The message is basically telling you that synaptic was not found in the YUM repositories.
With fedora you have
fedora-updates
fedore-core
and something else.
You can add additional repositories to gain access to additional software.
You do this in your /etc/yum.conf file.
Do a search for how to do this.
You will have to download and install the package yourself. You can find rpms from
http://rpm.pbone.net/
Click on Advanced and you can select only your distribution to search for.
