danielomat: Hi there,
Thanks again for all the effort you have put into this. However, I couldn't get the Galaxy client to install DLC in OSX although it's fine in Windows. Anyway, I managed to download the files and install them manually, which eventually did the trick.
Happy new year and thank you very much.
JudasIscariot: Could you tell me what happened when you tried to install the DLC for Master of Orion via Galaxy?
After un-checking and re-checking the other races the client downloaded the 3 files but didn't install them. Oh, and I found another quirk: I cannot start the game with a double-click, unlike all the other gog.com games I also own. Master of Orion only works through the client or when I open the App-folder and navigate to the executable (contents => resources => game => MasterOfOrion). Weird...
BTW, I'm on Mavericks, maybe that helps.
UPDATE:
Odd - it didn't work. When I installed the DLC, I did it with a fresh install from the website onto an external SSD from which I ran Sierra, and there it worked. Now, in Mavericks, on the internal Superdrive, I can't get the DLC to work. A fresh install from without the client, however, fixed the double-click problem. Will check permissions and integrity and see if I can find something...