You got about 4 areas that can go wrong. First is the shifter, not much to take
apart, pull the "U" shaped retainer from the front of the knob. Remove the knob
and check to see if anything is obstructing its movement. I've seen some that
the spring will break and get into the works and bind it up. If so any hardware
store spring of the right size with good pressure will work. Shifter number is
12562753 or 12564519 for the Z4Z option, Pace car. If not then you need to
remove the consul cover and then remove the shifter and check it out. Lots of
cheap parts inside to go south and I don't have a good picture of them, just a
general view of the assembly. If it's not there check out the cable next, you're
going to need to get the car up so you can get under it. If it's not there check
the lock out switch # 12450016 to see if it's broke and  restricting movement.
Last, but not what you want to hear, is the transmission its self, dealer time.
If for some reason it's the parking pawl it can be replaced without removing the
transmission.

Not much help but then I have 173 pages of diagnosis and 2 for the mechanicals.
;-))