Rachel, did you figure it out?
First you join the challenge, here:
Then you upload your layout in the gallery like usual. After you choose layout/project/card, etc. a drop down selection will be on the right side - where you put the name of the layout, etc. Scroll down and there will be list of all the challenges you are in. Check mark the one you are entering. After you are done, you can go back to the link I gave you and see your layout/tag in that spot with everyone else. Hope this helps. Let me know if you have more questions.