You need to find a voucher in one of the participating store.
I know there is some at Walmart for sure.
Look for the Air Wick Scented Oil Starter Kit cardboard display.
The voucher is stuck on the packages.
I hope those pics are gonna help you:
Attachment 3909
Attachment 3910
*You must by before December 31, 2008, but you can send the request until March 31, 2009.