The Twilight Hack is described at Code Retard. It works by using a bug in Zelda - Twilight Princess. In short is goes like this:

1. get WAD Installer 2.1 and copy the wad-installer.elf to the root directory of your SD-card and name it boot.elf
2. get the Twilight Hack Beta (for the Wii 3.3 firmware) and copy the rzdp.bin as data.bin to /private/wii/title/RZDP (P for PAL).
3. copy all wanted games (*.wad-files) to a directory /wad on your SD card (4MiB ~ 59 blocks)
4. get Zelda - Twilight Princess, run it at least once on your Wii to create the savegame slot
5. insert SD card, delete savegame on your Wii and copy the Twilight Hack savegame from your SD card
6. now run Zelda, load game, walk towards the guy and talk to him
7. the screen goes black and shows the WAD Installer which installs all files found in /wad