Problem with wireless flash on ist D

Just bought the 540 flash. It works fine on camera, but have had no success in wireless mode. The D is supossed to be able to trigger an off camera flash like the 540 using its built-in flash.

I believe I have set up the flash properly--channel 1, power switch set to "Wireless", P-ttl mode. Then I manually activate the built-in flash on camera by raising its hood. But the built-in flash just fires normally without triggering the 540. Is there something else I have to set on the istD so that it knows to signal the 540?
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ist D, CP5000,
Leonardo