AFAIK, all of the Steam pre-orders are bugged and none of us can have Imperial Edition content.
And one of the developers posted in the Steam forum that they were going to (most likely) update the game with an option to toggle golden dragon skin on and off.
Do you not have a "Imperial edition" Folder in your Divinity dragon commander directory?
Navigate to your Steamapps\common directory (Probably either "C:\Program Files (x86)\Steam\SteamApps\common or C:\Program Files\Steam\SteamApps\common" if you haven't specified an alternate install/library location"
You should see a folder for Divinity Dragon Commander and in that folder there should be the "Imperial edition" folder that contains the extra pre-order content.