my understanding that if you register using the packaged slip then it's instant, and if you purchase it online it can take up to 24 hours (providing your Wii u is connected to CN first). I don't know about the MK8 bundle, but I downloaded it the night before and the promo was there in the morning.
Club Nintendo is up and down and up and down but it will take about 24 hours in most cases before you are eligible for your free game. When you finally become eligible, you will get a pop up on Club Nintendo in which you can pick the game you want. Screenshot the code just in case the site goes back down but it should also be sent to your e-mail.