I have tried it with the link and used the activation key. The version I already have on steam was new installed (so all saved gams were deleted - but that's not the problem). So I have the basic version of midnight ride (without any DLC). If I try to buy a DLC (on the steam side or over the Steam Database) I get the info that the game is not available in my country (region blocked). If I try to use the claim steam key for galatic detour I also get no chance to use / insstall this DLC.
I do not know what I could do more as wait until the new DLC (Storyteller) is available on this side here. :(
Otherwise you send me detailed instrunction notes how I have to use your steam codes / install the dowbloads from your side that I can use it in Steam.