I'm with you, Lori--I find Designer Digitals' site set-up to be a lot different from many I've used. And they seem more restrictive (or maybe it's selective) about their forum.
Anyway, maybe this will help:
Basic posting guidelines:
Gallery + Forum Guidelines - Digital Scrapbooking Community - DesignerDigitals
They don't seem to have any challenges for December--don't know why. But I find them using the "Community" tab on their home page:
Challenges: Digital Scrapbooking Community - DesignerDigitals
I do like the pages I see there, but I've only posted there once or twice.