Inside of Invisible Infrastructure we teach you how to use a custom domain with your funnel.
Please be very diligent when going through this level and take your time to go through each move very carefully. This is one level where you may want to watch the video twice.
UPDATE: As of 2017, ClickFunnels offers a more automated way to use custom domains with your funnel. It's easier than the manual integration we teach, so if you're still having trouble you may want to follow their official guide here.