Same issue here. Followed the wiki verbatim and nothing seems to work. Added a DLC folder, put the rpa file in the game folder, made sure the base game and dlc were the updated versions, etc.
Try to launch the game without the dlc first. Then move the dlc back into the correct folder.
You can check the content of "Mobile search paths" in the file log.txt ( https://wiki.andrealphusgames.com/wiki/How_to_add_DLC#DLC_still_doesn't_show_in_... ) the DLC should be in this folder.
I'll try to find a way to ease the dlc integration process.
Update your main game to the latest version (25.3.0e).
You will find an import button in the option of the game.
Select the location of the DLC RPA file (downloads probably) and wait until the game restarts by itself.
There's a black screen during restart, I'll change that when I find some time.
You can check the wiki page ( https://wiki.andrealphusgames.com/wiki/How_to_add_DLC#Ingame_import_button_(since_25.3.0e): )
