Using a fresh install of elementary OS Freya
I installed Synaptic and expected it to show up in the applications menu. It is not there. I am able to start Synaptic by typing the name in the search bar at the top of the icons. I can't find a way to display a full list of all my installed apps (including Synaptic).
I installed Synaptic too and it shows up as Synaptic Package Manager in Slingshot (under System Tools if you use category view). By default it uses quite an ugly icon, this is how it looks like for me (with an extended icon pack and unstable WingPanel):
One idea would be to check the synaptic.desktop file in /usr/share/applications/. It should look something like this:
[Desktop Entry]
Name=Synaptic Package Manager
GenericName=Package Manager
Comment=Install, remove and upgrade software packages
Exec=synaptic-pkexec
Icon=synaptic
Terminal=false
Type=Application
Categories=PackageManager;GTK;System;Settings;
NotShowIn=KDE;
X-Ubuntu-Gettext-Domain=synaptic
Quite important are the entries NotShowIn (should not contain Pantheon) and OnlyShowIn (if it's there, make sure it does contain Pantheon). There should not be an entry called NoDisplay. (Learn more about recognized desktop entry keys.)
If there is no such file first try to reinstall synaptic sudo apt-get install --reinstall synaptic.
If that does not work, use Scratch to create a new file called synaptic.desktop inside ~/.local/share/applications/ with the contents I have posted above.
.desktopfile in/usr/share/applications.