Purchased the H850XP a few days ago but the FMS never it loaded data  and only read "loading data" which make the plane pretty useless on a flight greater than 30 mins. The manual is next to useless so am I missing something or is this a common problem.

Im enjoying flying this plain using the heading bug but need to get full navigation. Any help greatly received.

 

Had same issue with "loading data" for FMS.
Googled and my solution was editing the dll.xml file in folder
C:\ProgramData\Lockheed Martin\Prepar3D v4
(please note that the ProgramData folder is a hidden folder)
Then edit dll.xml file (always make a backup) by adding an entry like this:
<Launch.Addon>
  <Name>Carenavigraph</Name>
  <Disabled>False</Disabled>
  <Path>.\Carenavigraph.dll</Path>
</Launch.Addon>

Save file, done.
HTH. 

BTW: Found solution on Carenado's site.