@YoYo, the full path to the Doc folder is "D:\Program Files\Eagle Dynamics\DCS World\Mods\campaigns\OH-58D - Through the desert dust\Doc". I also tried placing it in "C:\Users\Cubicle\Saved Games\DCS\Mods\campaigns" but this did not change anything. At least, with the fixed entry.lua, the authorisation error message is gone.
When I place the campaign folder in "C:\Users\Cubicle\Saved Games\DCS\Missions\Campaigns\en", I can see the campaign description and image. But clicking OK doesn't start the campaign. I can see the throbber for a short amount of time and then nothing.
EDIT: @YoYo, I found the issue. The first mission requires the Marianas map to be installed. Without the map you get no clear error message. It's just not working. Perhaps you can update the manual or the description to let people know.