You need to browse to the '..\SteamApps\common\Divinity - Original Sin' folder if the support tool doesn't find the install path. It automatically adds the '\Shipping\EoCApp.exe' part of the path, even if you browse to the Shipping subfolder.
If you are not sure where it is installed, in the Steam Library right click on Divinity Original Sin, select properties, click on the Local Files tab, then hit the 'Browse Local Files...' button. Windows Explorer will open to the 'Divinity - Original Sin' folder.
