Well you have the normal releases with red push buttons in the middle of the row on the seat part and the other ones are on the outside of the bench on both side, the black release button is on the inside on the buckle not the outside but it is fairly easy to acess, it is just a push. Wished I took my camera along, I will try to stop by at the dealer again but it was a customer car which was due to be delivered shortly.
C

here is a pick of the outside buckle....cant find a release button

X5FX, as clear of a picture as I have ever seen, you are a gifted photographer.
That's what I suspected it is a different buckle. The EU model has a normal saet belt latch release with a button, not dissimilar to the one you find in the center of the row. I will try to stop by my dealer to take a picture.
It must have to do with different safety laws in the US versus EU hence explaining the fact you are not able to release it.

C

I was reading this old thread and figured I'd add some info for the archives.

As I was just looking into this I found there are 2 part numbers for the 3rd row seatbelts. One with and one without the release mechanism for stowing. As speculated earlier in the thread, this is probably due to restrictive US regulations.
RealOEM.com * BMW E70 X5 3.5d Safety belt for 3rd seat row

I suppose you could retrofit the stowable European version of the belts. It would be a little expensive at ~$120 per side. ...and I'm not sure the US C-pillar bezels are designed to have the seat belts clipped in for storage, although I can only find one part number for it so maybe so.

I'm going to look into it and will post back if it seems worth doing. I'll have to find a better price on the belts though. At $240 for a new set it's not worth it to me. Maybe I can find a cheap set from a salvage yard.