FOLLOW-UP:
I ended up purchasing a Visonic K-940 PIR detector. It works like a charm. You must either have a separate 12VDC power source for the detector or (a I did) derive the power source directly from the PCB of the DCS6620G - I am sure D-Link will keep the warranty valid

In summary, you will have 4 wires coming out the SENSOR. A pair going to the 12 VDC poser supply, the other pair going into the DCS camera.
Unfortunately I ended up wasting hours and risking to burn out the sensor because the wiring diagram on page 134 of the DCS-6620G is incorrect. It shows + and - of the PIR sensor going into PIN 3 and 4 of the DLINK DCS6620G. DON'T DO THAT! Instead of 12 VDC pin 3 and 4 are at 24 VDC... and you risk to burn out the detector/damage the DCS. Just read the instrution carefully and do not rely on the diagram.
ANOTHER SUGGESTION TO D-LINK: It would be really great i you guys could embed a PIR sensor in the next detection internet cameras. It would be extremely economical and straightforward to implement and would provide a significant added benefit for motion detection.
Cheers,
Fabio