The 'Don't Starve' DLCs (at least 'Reign of Chaos', but I think 'Shipwrecked' as well) on OSX are broken. Problem is the main programm ('Don't Starve' without DLC) is an application bundle within an application bundle (second layer). Whereas the 'Reign of Chaos' DLC is installed in the first layer. So the main programm can't find the DLC. To fix it you have to extract the bundled main bundle (in contents/resources/game) and copy the DLC folder (in contents/data/) to this new bundle.

I hope I described it good enough. If not feel free to ask.

Good to know. I have no way of personally testing this at the moment, so may I recommend getting in touch with support?

I informed GOG-Support and they made a new OSX bundle. Problem is fixed.