(synaptic:6761): Gtk-CRITICAL **: IA__gtk_widget_hide: assertion `GTK_IS_WIDGET (widget)' failed is not an error but a harmless warning and would not prevent Synaptic from starting. Synaptic is run through console helper. If you want to run it from a root terminal then you need to put the full path to the binary. Why are you trying to run it from a terminal anyway? If you want to explore how synaptic starts then look in /usr/share/applications at the desktop file.
I was trying to run synaptic from the terminal because in E17 synaptic asks for root privileges and doesn't run. If i run it from the terminal it opens and i have the Gtk-CRITICAL. I didn't know that this is not an error.
Thanks.