Technically you need to buy it new, not used. The phrasing is "In addition to the Apple-branded product and accessories contained in the original packaging, Covered Equipment includes the following according to your Plan:
(i) Under APP for Mac, ... (c) ... MacBook Air SuperDrive if used with the Covered Equipment and originally purchased
by you no earlier than two years before the Covered Equipment purchase."
Source:
http://images.apple.com/legal/sales-support/applecare/docs/081811_APP_English_NA_v5.4.pdf