Read the Readmii and then download the v6 MKW Hack Pack in the Official Download folder.
You don't have to download the Master Code Pack, as it is optional.
Open up the zip file
Put the apps, MKW Hack Pack, and riivolution folders on the root of your sd card. The above picture shows you how your sd card should look like if done correctly. (note, my sd root maybe named differently than yours).
If you have any files ending in .elf or .dol on the root of your sd card, delete them, as they will interfere with MKW Hack Pack loading.
This is how your riivolution folder on your sd root should look like if done correctly. If you have any riivolottest xmls in this folder, delete all of them, along with codehandler.bin if you had it before.
Also, if there is a folder named save in the riivolution folder, keep it, as that contains your save game for MKW.