before you do everything, just make sure dont login eshop, dont turn on the wifi. means dont update your 3DS. 3DS has been updated to V4.3.0-10. Acekard 2i can not work on it. so just make your 3DS stay on V2.1.0,
At first. make sure your ak2i is not a fake one, the real ak2i should be looking like this : buy Acekard 2i flashcart for DS DSi 3DS-dsflashcart.com
If your acekard 2i is still working, here is the guide to use it
Step1: Download the latest AKAIO V1.89 from here: http://www.acekard.com/download/akaio/AKAIO.1.8.9.zip Extract it and put all the files or folders in the root directlory of your sd card. (suggest you format your sd card before you use it on 3DS)
Step2: Put your game roms in the sd card.
Just insert the ak2i and the SD card into the 3DS and turn, enjoy it.