Where is the pound mark on keyboard?

On English keyboards it is on the same key as the number three so you would use the command 'shift' '3'

In a Windows environment if your keyboard does not have a £ sign you can try using the Character map from the system settings or

With the NumPad active (a little light should indicate that it is active - there is a NumLock key on many extended keyboards) Hold the left Alt key down while you press (on the NumPad) 1, 5, 6, and release the Alt key.