Here's a link to the first ATC Challenge thread:
Forums at Splitcoaststampers
This thread gives you more information about the challenges. Basically, though, this is just a chance for us to try something a little different, to stretch ourselves just a bit. We don't have sub-forum for the ATC Challenge threads. Perhaps if we have a few more people interested and participating we can get on. If you put ATC and then the number of the challenge (ex.: ATC5 for this week's challenge) in the keywords space when you post your photos, then others will be able to find them more easily. And if you want to see other's ATC challenges, you can go to the gallery and search for ATC and then the number - 1 to 5 so far.
I hope that helps a little. I'm looking forward to see everyone's art posted in the gallery.
Teresa