the cable should fit into a small slot at the end of the throttle grip that goes half way round the base of the grip. it sort of feeds round and the knobbly bit at the end of the cable fits into this and slots into the end. It fits similar to a brake cable on a push bike, sort of! thats for visual purposes! However if it has come off I would suggest that the throttle clip that the cable fits into may be worn or broken. Hope this helps a little. It is very simple just take your time you should see where it goes. If you have a Haynes manual this will show with piccys.
bb, April 2010