It seems to install now, at least on the version I got with the game. I bind it to my account by entering the serial number. However, when I start my newer 1.5.4 version of DCS there is no window popping up that tells me I can install FC3 now, which bugs me a bit because I have to play it now with an old version of DCS with no chance of an upgrade at all? If that is true, it is really punishing those who buy the physical copy.
Edit: Ok, false alert, it installs also on the 1.5 version. Thanks for help. :)