If your DVR is connected to your network and you are running media server software on your DVR (HR2x or R22), there will be a new menu item, "Music, Photos & More". Selecting that will show you a list of computers which are running a suitable server. If you select a computer, you will see a choice of Movies, Music and Pictures. Only specific formats are supported by the DVRs:
Movies - MPEG2
Music - LPCM
Pictures - JPEG
Most server software will do some level of transcoding of other formats (such as converting MP3 audio to LPCM), but different servers have different capabilities, particularly with video formats. Suitable servers include:
Windows - Tversity, WMP11
Linux - MediaTomb
OS X - EyeConnect
There are others, but these seem to be the most widely used servers.