MSFS ATR RNAV Approach
First of all, when you creating your FMS Flightplan you must select a suitable RNAV Approach for your Destination Airport.
In the according charts you will find the MDA barometric height for your approach.
Turn the DH/MDA switch to MDA and set the Altitude to 400. Its important to set MDA on both sides, Pilot and CoPilot, otherwise MDA will keep flashing.
Now depart and climb to cruise Altitude and relax until ToD. Short before reaching ToD, set the Altitude to 2000, which is the FAF for the example above.
Wait for the vertical path to be calculated by the FMS , then you can see the Magenta indicator on the PFD.
Now you are able to pressVNAV button. Without vertical path calculated and indicated by the FMS, VNAV will not work. If the waypoint for FAF has an height assigned in the FMS, this will be shown in Magenta above the VNAV Indicator. Magenta means always, calculated by the FMS, blue means input from Pilots.
The PFD wil now show
Waiting for the chime, indicating that you reach the altitude of FAF.
Now dial in the MDA Altitude and press APP in FMS Mode (not V/ILS).
V-FP will NOT engage as long as you follow a VPATH (dont know if this a bug or a Feature 
The PFD wil now show
As the ATR has no Autoland, when reaching MDA and the Runway is visible and clear of obstacles, disarm the autopilot and land.
Hope this brief description helps for successfull landings.
Chris