DVR is usually connected to the router. And then a computer is used to find out the IP address of the DVR. I have never used the Airport Extreme, so can't help you here, so can't help you to find the IP address or to find out how to enable port forwarding. May this is impossible. Next time make sure to learn about hardware before buying it, don't just buy it because it is made by Apple.

Every router is different. And there are so many of them, that they simply cannot be covered here. May be some users share their experience, but you shouldn't expect immediate replies. This is not a paid support site, this is just a free forum.