I've tried it. Recived the same "no entry" error. I have also reinstalled the game.

Well, that was emberossing. Sorry.

Well, I thought I've once seen an add when Arma 3 was Alpha saying that anyone buying the support edition of the game will recive, in addition to the game itself, a steam version of Arma: Cold War Assault, some Arma 3 music and the 3D editor. I was not aware however that they plann to make a better-than-in-Arma 2 3D editor after 2015 so, thanks for thi intel.

Is that official or is it community based? I didn't know about it's existance (if it's official). Thank you for voting

That's verry important in my opinion.