Hi!
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.