There is a video on how to open the back cover of your Nokia N950: https://www.youtube.com/watch?v=xWcGOe8lfe0

What CLI flashing tool are you using?
The one provided by Nokia for N950 will not work if you're on PR1.3.

Try getting the flashing tool for N9 and run "flasher -i": http://wiki.maemo.org/Flashing_N9
Obviously don't flash with N9 ROM! (Just a remainder)