You must ensure the DLC region matches your base game. If you have the North American version of Stretchmo, you need the USA DLC CIA; otherwise, the game won't recognize the extra levels.
Always ensure the base Stretchmo app is installed before installing the DLC CIA. Even though the base game was free, the DLC acts as a "key" to unlock the levels. Stretchmo 3DS (USA) DLC CIA
Subreddits like r/3dspiracy have extensive megathreads and guides on how to safely find and install DLC for games that are no longer available for purchase. Safety and Requirements You must ensure the DLC region matches your base game
Because the eShop is closed, new players can no longer buy the DLC officially. This has led many to look for archived CIA files. you need the USA DLC CIA