You need to open/forward ports necessary for your DVRs service to be available through the routers NAT firewall.
For example, if your DVR is accessed via a web interface, perhaps that runs on something like port 8081..(you'd have to refer to your documentation or DVR support). You create a static LAN IP for your DVR, such as 192.168.1.50....most routers support this via DHCP reservations. Then you open/forward port 8081 on your routers port forwarding section of the web admin to that internal IP address.
Now when accessing from the outside, point your browser to http://my.dyndns.org:8081 for example.
Or does your DVR run on a host operating systems, like..runs right on Windows XP. Then you'd probably just want a remote desktop viewing program, like logmein.com free.