The metal box in the wall is called a pattress I think. If you have an Earth wire it should connect to a small brass screw terminal in the back of the pattress.
On the Switch, the Live arriving at the switch always by convention is connected to the common terminal (Com).
The live that gets switched on and off (the other wire) can connect to either L1 or L2 it really doesn't matter.
Under no circumstances connect an Earth wire to the switch - else you'll get a mighty Bang!
As plastic components are non-conductive you will very seldom find an earth connector attached.
Take care...
Peccavi
May 2009