It helps when you read the manual, you know. Anyways, you can only register FC when you're online, and for you to be online, you must have a wifi access point set up properly on your 3DS. So go setup a wifi access point, go set up the access point on your 3DS, then go back to the FC app and wait for it to go online (your Mii profile will go orange instead of just gray), then try adding.