I agree you can play music froma USB stick connected in the glovebox or the arm rest.

I did notice that on page 118 of the x5/x6 manual it references the USB connection in the glove box. There is no * on the title which means it is not an option (assuming the manual is correct).

On page 183 where the reference the USB connection in the arm rest you will see the * indicating it is an option.

I don't know if the manual for the car is definitive but this with what I was told by the dealer.