Why does my GFCI receptacle trip when I connect the load wires?
It sure sounds like you have a ground fault on one of the down stream outlets or devices. Check the load side devices for proper wiring etc. Hook the circuit up without the GFCI and use a plug in tester to determine if any devices have reversed hot, open neutral, etc. Also check to see if there are any motor loads on the load side circuits, motors can cause nuisance tripping.