HELP fitting of a towball on a discovery 3

[H 8 ROK]

Just bought a tow ball of eBay for my discovery 3, it has keys in the tow ball and i need to know how to fit it

any advice would be very helpful

[Gazzab]

It is all explained in the handbook.
You need to have the green bit showing, click it in place and then the green bit disappears.