Check this guide:
http://steamcommunity.com/sharedfiles/filedetails/?id=276611375"WARNING:
Opening the editor and Main story mod will spoil the game! Do not open the editor/Main mod if you would like to finish the game without spoilers! Do not save when the main mod is opened. This can break the game and data/levels!"
No need to re-install the game. Just delete the folder ...\Divinity - Original Sin\Data\Mods...
You need to 'Mod' the main campaign, not alter it directly. At the moment when you create a new mod and select Main as dependency it show no levels, this will be fixed today.
When I create a mod using Main as a dependency and a data pack, I am able to open all the levels from he main campaign. However, I am not able to modify anything (control+b just shows grayed out text for any entity). I am able to add new things to the level however. For example, I can add a tree and modify it, but if I try to modify a tree that was already placed in the level by Larian, I can't seem to be able to do it.
Is that a bug that will be fixed today, or am I just doing something wrong?
Edit: Also Publish to Steam Workshop feature doesn't seem to be working. Yesterday it was giving an error, today the button is just grayed out. What's going on with that?
By the way, love the editor and the tools you guys are giving us to make our content
