I bought from gamersgate store which is steam key.
I had report this in steam's fantasy general 2 forum but I think due to different time zone, so currently no staff answer my latest thread at there.
At steam I already get an update for fantasy general. (I think it is 400++MB update) that why when I go in game, I can see new dlc in game menu but when I choose at there, it ask me to "buy dlc".
I had restart steam. I can't get the "dlc management" button at my game library because it assume that I not owning any dlc.
When I go to "support" -> "dlc or bonus content missing"->"dlc", it show that "You do not have any DLC for this game. Please double check what is included with the version of the game you have. If you still believe you should have extra content for this game, please check your purchase history to verify you have made a purchase for DLC for this product."
I try register the steam key at slitherine register a product page but it not accept it. I think it only accept matrix/slitherine own key rather than steam key.
Here I upload my license to show that I own General Edition
This is the steam dlc page.
If I own the dlc, it will show as "Fantasy General II: Onslaught is already in your Steam library"
but now as you can see, it still show that I not own it.
I hope can solve it without much problem because I going to buy Panzer Corps 2 Field Marshal Edition in this few days before it release. This one also from slitherine and include 2 future dlc. Same as this FG2 General edition.