Train Simulator: DB BR 642 DMU Add-On DLC

I have been playing this add-on for many many hours when I realized, that it has a different file structure than the original one from RSSLO. I have downloaded a freeware mod to this add-on and installed it as in the manual, then I was unable to insert the key, no matter that I clicked the right spot. I tried it 5-10 times and the conclusion was the same: without the freeware add-on it worked, after that it was not possible to insert the key, therefore the whole train was useless. After buying the same train from the RSSLO website, I installed everything just like before and voilá it worked. The only thing I can think of is, that somehow this version has a different file structure therefore it is not compatible with add-ons. I described the situation to Steam support and asked for a refund, because I had to buy technically the same thing twice - I got rejected because I played more than 2 hours. Of course I did, but they sold sg which is different from the original copy. I bought the train here, because it was on sale, now I know I wasted that money. Ever since if I can buy sg here or elsewhere, I do it elsewhere. It is a shame, lack of support as well. Not that the price I bought would get me bankrupt, but I really don't do it because I got bored with this add-on and want to get my money back, it simply differs from the original one - which I again bought after I learnt it. I am disappointed and I can advice all to buy the add-ons from the original sources not Steam to avoid such happenings.