DVDX is a homebrew application that installs a hidden "channel" which can be used by other homebrew applications to access home burned and factory pressed DVD content without the need for a modchip. This does not modify any IOS in any way, making it very safe.
Installation is simple - just run the DVDX stub installer (Pre-Oct 23 update for system menu 3.3 and older, or Post-Oct 23 update for system menu 3.4)). If you have a modchip, you may need to install PatchMii core before installing DVDX. Then, when asked for the installation mode, select PatchMii. Please note that the post-Oct 23 version does not have the PatchMii and advanced modes. That means that some modchip users cannot install it.
Q: What type of DVD should I use?
A: Pressed DVD-ROM video discs of course work. For recordables, DVD-R media is recommended. However, DVDX has been confirmed working with various DVD media, including DVD+R/RW and DVD-R/RW discs. Particular brands (lower quality discs) may be problematic, and different Wii drives have different levels of compatibility. Please note that DVD+R/RW discs will need to have their BookType set to DVD-ROM to work properly in Normal mode (if you have no modchip). PatchMii mode does not have this requirement, but requires a modchip.
Q: Why does my Wii not read my DVDs?
A: Be sure that you properly install DVDX, and that the application that you are trying to use supports it. If you are using a burned disc, make sure it is burned correctly. Try re-burning the DVD on a DVD-R if it is not already. If you are using one of Tantric's apps (the GX emulators), try using an older version, as many times in the past he has accidently broken DVD support.
Homebrew developers looking to add DVD support to their applications will need to use libdi, which has been added to libogc. libdi requires the DVDX stub.