There's a link to the coupon generator in their FAQ (and presumably elsewhere); I can't see anything useful since I have no books published there.
The more complicated part might be getting your book(s) formatted for Smashwords. But if so far it's only available for Kindle, it should be worth it, since limiting your book to only one format is a bit sub-optimal.
Smashword requires .doc format. Here's their style guide:
http://www.smashwords.com/books/view/52