Ok its sort of complicated unless you have a firewire on the back of your DVR which makes things very easy. if you do, you can just use the firewire cable and hook it up to your PB and import it into iMovie and convert into whatever format you like..
If not, then you need something that can capture that video signal into the computer. In the case, you should look into something like
Elgato.
The best home-brew solution .. in case you dont have a firewire output or dont want to buy any more equipment is the AV camcorder trick.. Connect AV cables from the back of your DVR to your cam, then get it to iMovie.. and voila!