I've noticed a lot of people are having trouble finding out how to file one, since DA doesn't exactly make it easy to find on the site.
The link to the form is here - http://help.deviantart.com/dmca
~~Information about DMCA~~
This form allows a copyright holder or authorized representative to file a takedown request pursuant to the Digital Millenium Copyright Act under United States law for his/her/its copyrighted content placed by a third party onto DeviantArt.com without permission or other legal authorization. This online form allows us to quickly and easily gather the necessary information to assist in such requests. A takedown request