First of all you have to download the 3DEN plugin from the link below:
After that create a folder in your game directory and name it whatever you want to (For example: "@ExileEditor") and simply paste the content of the downloaded file into the created folder and load it via the ArmA 3 launcher. (Copy the addons folder inside the archive and paste it into your new created folder or you can just change the name of the "@Exile" folder inside of your downloaded archive to something else.)
Now simply open the game up and start the 3DEN Editor and you'll see a new tab named "Exile".
For more information about how to implement your created objects into your mission file and etc, take a look here: