To clarify, a is not actually a file "converter" in the traditional sense . A .7z file is a compressed archive (like a digital suitcase), and an .nds file is a Nintendo DS ROM typically stored inside it.
If you have successfully extracted the NDS file but it won't load, ensure your emulator is up to date. For original hardware users, make sure your flashcart (like an R4 card) firmware supports the specific game. 7z to nds converter