Why is my GFCI receptacle tripping when I add another receptacle to the circuit?
Look for a ground fault

Disconnect the wires feeding the new receptacle, at both ends.
Set your multimeter (you do have a multimeter, right?) to the lowest resistance setting, or the continuity setting if it has it.
Touch one probe to the green/bare grounding conductor, and the other to each of the other wires in turn.
repeat this process for every combination of wires (G-&gt; B, G -&gt; W, W -&gt; B).

With the wiring disconnected, you should not get a reading on the multimeter.  If you do, it means you have a short-circuit between that set of wires.  Repeat this process on the new receptacle, touching one probe to the green ground terminal and the other to each the brass and silver terminals.  Again, no combination should give a reading.
Your circuit is too long?
It may be possible that the circuit is just too long, and the resistance in the conductors themselves are causing the imbalance. Try connecting the new receptacle to the previous receptacle temporarily, using short pigtails (CAUTION: Working on energized circuits is dangerous.).  Turn on the power, and try to set the GFCI. Make sure nobody is near the exposed receptacles, and don't touch, or get near them while the power is on.
If the GFCI holds, there is a fault in the wiring to the new receptacle or the circuit may simply be too long.
Warning: I haven't done any calculations, research, or testing on this theory, so it could be completely false.  It's just a thought, but it's fairly easy for you to test.
According to the documentation for the Two-Pole QO®/QOB Circuit Breaker and Ground Fault Circuit Interrupter

NOTE: To minimize nuisance tripping:...

Do not connect circuit breaker to  more than 250 ft. (76 m) of load  conductor for the total one-way run


Is the power on?
GFCI receptacles can only be reset when the power is on. So if you're trying to reset it with the power off, you won't have any luck.