You must be more specific when you talk about DVR. A TiVo DVR this would be easy, with nearly all others, it is much more difficult. The easiest way is to record this onto a VCR or DVDR system for the TV, as most companies usually lockdown their DVRs to prevent piracy (because they have no encription on their files). If yours does have a Firewire port, it is possible that if you connect to it, it will either show up as an external HD or a Video Source. If the former, it is just a matter of finding your video, if it is the latter, you should be able to capture it in QuickTime (or iMovie), by creating a new recording and the playing it back on the DVR, the video will then come across the Firewire like a DV Camera.
Good Luck,
TEG