Weird. On the Air I'd try going to Safari Preferences, click on Privacy, then click the Details... button underneath the Remove All Website Data... button. On that next screen enter 'digitalmediatree.com' into the search box to filter the list down to just DMTree cookies, and then click 'Remove All' at the bottom to get rid of your DMTree cookies.

If that works on the Air I'll figure out how to do the same on the iPad. Not sure if that will work though.