The contest is open to anyone living in the U.S. (BorrowLenses.com only ships to U.S. addresses and only offers in-person pickups in San Mateo and San Jose, California). All you have to do to enter is post a comment [Ed. on DPS' website - link] with the lens you want to rent. A list of lenses is available on the BorrowLenses.com website and the selection seems pretty decent. The winner will need to provide BorrowLenses.com with a credit card number, ‘just in case.’
You can enter on DPS’ post announcing the contest.