Collect Additional Customer Fields in MarketPress

So it would be nice if I could somehow customize the check out fields that customers fill out at the time of check out. Can you show me what files I have to edit to achieve that? The nice thing about wp-eCommerce is that this is possible by going to their back end and setting the custom fields there, never had to touch a bit of code to get the results I wanted. If there is any way I can get that done I’d really appreciate guidance!

Thanks!