I've got a new arc fault circuit that trips immediately. I've switched out the breaker with the same results. How does one locate the problem in the circuit?

I may have answered my own question:

http://www.arcfault.ca/www/promo/0760DB0204.pdf

But I"d still appreciate any expert insights.

Have you looked for any frayed cords or a shared neutral? Probably the most common causes. There may be others but need to establish the condition of the existing wiring.

If it trips immediately with load wires disconnected, Bad Arc Fault. If you are adding Arc fault to where regular breaker was, may be sharing Neutral. All Hots and Neutrals must be on Load Side of ARC Fault. If neutral from arc circuit paired with porch light switch leg for example, it would trip, They work similar to GFI's.
Your situation: remove arc fault breaker, unplug everything, and turn off all switches, for that circuit.
You should read infinity at panel. Resistance between hot or neutral to ground or another hot needs to be located and corrected.

Thanks guys, I did get it figured out. That PDF I linked is REALLY helpful for troubleshooting these puppies. Ended up being a loose line terminal on an outlet.