If you want your book available for free on BN.com you need to publish it through Smashwords(or possibly Lulu.com though I don't use them so I'm unsure).
Forewarning, Smashwords will screw up your book, though. They only send the short description to B&N, and they have been having recent issues with cover art missing when the book is ported over from there to here. Also, it will potentially take weeks to do, so you can't really plan for a sale or marketing thing.
It's really a huge hassle and I wouldn't suggest it, but if you want to do it still then there's your option. $0.99 is practically just as good, and it's easier to do at every eBook retailer.