The Files on the SD card can be accessed from two locations on the Wii: The Photo Channel, or through the Wii settings.
To find the audio files on an SD card, you should go to the music setting for the slideshow, all music on the SD card will show up with the built in options. The video files should show up easily when you select the SD option on the Photo Channels opening menu.
To find all files on the SD card at the Wii settings, select Wii settings, then data, then SD card. Pretty simple.
hope this helps!