MarketPress -- How to add "referred by/how did you hear about us" to checkout

I'm using MarketPress on an ecommerce site. My client would like me to add a question to the checkout process.

How did you hear about us? Friend, Social Media, Search Engine, Other

How can I add this to MarketPress? I'm sure the answer is already in the forums somewhere, but I've been looking for an hour and can't find it.