Are you confused yet?
I think what you have is 3 pieces of romex coming into the ceiling box for the light. Each piece of romex has a black, a white, and a ground (green or bare) wire in it. One piece goes back to the circuit breaker and provides power to the circuit, another goes to the switch so you can switch the light on and off and the third goes to the outlet.
The only diagram I could find is this one.
http://www.buildmyowncabin.com/elect...nto-light.html
It doesn't have the wire for the outlet on the diagram.
To add the outlet, connect the black wire from the outlet to the junction of the black wire that goes to the breaker and the white wire that goes to the switch, connect the white wire from the outlet to the white wire that comes from the breaker and the white wire that comes from the light, and connect all the green and bare wires together.
The white wire that goes to the switch should have a piece of black electrician's tape on both ends so the next poor fool who looks in the box will know it is hot.
Any metal boxes should have a green or bare piece of wire attached to it by a screw and the other end of the wire attached to the junction of the ground wires.
The old fixture had extra wires for the outlet compared to the new light.
The piece of romex with the ground cut off probably is the one to the switch. The old style switches don't have a ground connection so it wasn't needed back in the day. The current convention is that the switch and the metal box it is in should be attached to the ground wire to ground them. You should pull another piece of romex between the switch and the ceiling box so you can ground the switch and it's box and put in a new switch with a ground connection on it.