None of the Nissan specific instructions look right for my 2000 Almera though many look tantalizingly close.
Mine has 3 wires in the original remote connector, it looks like the green (pin 20) and green/white (pin 21) are power and green/orange (pin 22) is ground.
Further up and past a connector these become red, yellow and blue wires respectively and from the interface instructions I suspect we should be using those wires rather than the green ones at the harness.
The buttons on the remote are two groups wired in serial with resistors between them. Pressing a button shorts to ground so I guess the original head unit must work out what button was pressed from the voltage it can send out out through pin 20 or 21.
From the interface instructions it looks like that works out what button was pressed by the voltage it can send out on it's white wire.
Based on the instructions for other models and if I had to guess I'd say we need to put a 47ohm resistor at the end of one of the original power wires and an 150 ohm resitor on the other original power wire, connect the other ends of those resistors to the interface's white wire and then ground the original blue wire to the headunit harness ground.
After that it *should* just be a case of wiring the interface to ground and 12v ignition power on the headunit harness, programming the interface then plugging in the interface jack plug to the headunit.
At this point, though, I'm just guessing - I'm no electrician and without some confirmation I'd treat my opinion with a huge pinch of salt (and thick rubber gloves).
I've mailed the manufacturer my guess at the setup and a schematic of the remote circuit, I'll keep you all informed of any reply.