My synaptic package manager is not launching from the application menu but i can launch it from terminal with gksudo synaptic. I have tried several times but I can't launch it from the application menu.
The Launch command is synaptic-pkexec (this is accessed from the application menu editor).
You can fix it by following:
Open terminal (ctrl+alt+T) and execute:
gksudo gedit /usr/share/applications/synaptic.desktop
If gksudo is not installed, you can just install it. It's provided by the gksu 
package.
(Or you can run sudo -H gedit instead. But avoid sudo gedit).
Change line Exec=synaptic-pkexec to Exec=gksudo synaptic.
Save file and close text editor.
I encountered the same problem, while I was trying to set up a new IBus input method for my newly installed Lubuntu 12.04.
I resolved the problem by purging and re-installing the IBus package:
sudo apt-get remove --purge ibus
sudo apt-get install ibus
My Synaptic Package Manager now launches again, its application shortcut 'synaptic-pkexec' remains unchanged.
