We're in the same situation - have the right holes in the seat, but the isofix "arms" wont lock onto anything. Can feel a metal bar, but that appears to be in the seat, and a bit too high (even when the isofix "arms" are raised slightly). Behind the seat, aligned with the slits are two holes in the frame...
Spoke to Nissan UK, and the dealership, about this; Nissan said "maybe", the dealership (Ancaster) said "No"
So we went for a maxicosi Tobi - seems to get the best ratings for a belted child car seat.
It could also have been a dealer fit option. I was able to find in fast the ISOFIX mounts for the 4wd Y11. Looks like one bracket for the middle two clips and seperate ones for the outside. I just can't find my way around the UK fast version to see what the N16 brackets are like.