First and foremost, you must have purchased Nevada (NTTR) or Normandy map, all of them can be installed to DCS 2.5
You can buy the map in our E-Shop: DCS Terrains
Once you have DCS 2.5 installed log on to the module manager in game, your purchased terrain should be available for download.
If not, please check your purchase is bound to your ED account here using the license check.
If you still cannot install Normandy map and Assets pack