Can't get it to work.

I recently downloaded the world ends with you. However, it comes up with "this game cannot be read, please turn off the DS and reinsert the game."I am using an M3 simply, does anyone know how to fix this issue?

Try reading this?

Aren't helpful, I really don't get it.

I suggest you read it over a couple more times. (Hint: The M3 is a flashcart)

I already got the firmware. It still doesn't work. I think I may have extracted it to the wrong place on the m3.
Not sure though.

Exactly as it says.
Upgrade to the latest firmware. If this does not fix your issue then your flash cart is incompatible with the game. Update the firmware by clicking Linfox Domain in my sig. If you are able to run your M3, you placed the firmware on correctly.