It is mentioned in several other places, but as this is one of the most frequently asked questions, here it is in an item of its own:
How do I get the password to enter?
If you are new and do not know the user id and password, or if the password has been changed since the last time you participated, please email the pass holders.
One of the challenge administrators will contact you with the password, but please be patient. There are only a few of us in different time zones. It would help us if you tell us your DPreview handle, and the name you will use to post your pictures in the galleries.