You can leave public comments for everyone who visits your contests, as well as private comments that can only be seen by the designer who created the design you’re commenting on.
Just click the Messages tab on your contest page and there is the option to Message all designers.
The Message all designers area should be used for general comments about the contest — anything you want to say to everyone participating in and watching your contest.
You can leave private comments and feedback for individual designers by clicking an entry which will show the design entry full screen. On the right hand side of the page there is a comment box where you can leave a comment that only that designer will be able to see
You can also view all of your private communication with a designer and continue to communicate with them by clicking the Messages tab on your contest page and then clicking the designers username.