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
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.