Where Friend Codes are concerned on the 3DS, Nintendo has decided to stick with the Friend Code system (as apposed to tags like on the PS3 or Xbox 36), but they have simplified it - only requiring one Friend Code for each system rather than one for each individual game, as was the case on the original DS and Wii.
Where do I find my Nintendo 3DS Friend Code?
1. On the Nintendo 3DS Home Screen, tap on the smiley face icon in the bar across the top of the lower screen. This will take you to the Friend List menu.
2. Scroll across to the image of your Mii, and your 3DS Friend Code will be displayed under it on the upper screen. The Friend Code is made up of twelve numbers - three banks of four separated with a hyphen, e.g. 0123-4567-8910
How to I add my friend's 3DS Friend Code?
To register the Friend Codes of others, click the 'Register Friend' link at the top of the the Friend Code menu.
Registering these codes will allow you to see when your Friends are online, and give you the ability to exchange short messages.
If you have any questions about Friend Codes on the 3DS, comment below and I'll do my best to answer for you!