I was at a nissan dealer and something similar happened, but this was a
2006 Quest, it was on exhibition and they needed to move it, but they
had removed the batery, while they were finding one available a
mechanic was trying to  unlock the shifter, and told me that at the top
of the slot were the shifter runs at the "P" position there is a
smaller slot that can be pressed with a screwdriver and unlocks it, I
don't know if this is the same kind of transmission, look for the
smaller slot, it may work.

Good luck

I have been having electrical problems and now my shifter is stuck in
park. The first time it happened two fuses were blown. One for the tail
lights under the hood and one for the Stop under the dash. The trailer
hookup in the back looks like the problem. There is a burnt wire where
the little black box is. The one that has wires coming in and out is to
the trailer. At this time no fuses are out and the tail lights are not
working. I need a Schematics. This is for a Nissan Pathfinder 2000. Any
help would be nice.