Since a lot of people seem to have trouble with this. I've uploaded a pack with all the necessary files to setup PastaCFW/RxTools Autoboot using Cubic Ninja in the link below. The instructions are pasted below. Simply copy all files to your sd card's root and follow them to have Autoboot setup on either an n3DS or an o3DS. I have tested it on both and it boots fine every time. Megaupload folder link below. Simply download the appropriate folder and follow the instructions. CakesFW will be...
[Release/Tutorial] HomeMenuHax Autoboot Pack (Easy Instructions)
[Release/Tutorial] HomeMenuHax Autoboot Pack (Easy Instructions)