Even without a modified system, you can now get DVD playback on your Wii . Mosey on over to HackMii where bushing and his band of cohorts have been working on the DVD Access library.
The DVDX installer instead will install a small, hidden, channel on your Wii that allows you to read DVDs on an unmodified system. It is not an installer for a patched IOS. You may however need one, depending on your system.
Team Twiizers also expanded the mplayer project to include playback of video files from the SD card. Check it out here for downloads and glowing commentary.