WiiMC is a full fletched Wii Media Center. It allows you to play video/audio from USB/SD and (on older Wiis) DVD.
1. Download WiiMC from the link above. If you already got it on your SD, get the Update pack, else get the "New Install" pack. Extract the Apps folder to the SD card.
2. All you need to do now is to run WiiMC with the Homebrew Channel!