I wish there was an easy way to just point it at a folder, but at the moment it seems that you have to manually handle adding the content. If anyone knows of such a solution I'd love to hear about it.
The IPFS daemon handles NAT in much the same way that BitTorrent does: NATed peers will dial out to un-NATed ones to establish a connection. I'm not sure if it also handles UPnP and the like to automatically forward ports if possible.