Nintendo Wii Operation Manual page 33

Removing the Controller Socket Cover
If you are using the Wii console in a horizontal position, you must remove
the controller socket cover when using GameCube Controllers to allow the
console to lay completely flat.
1. Stand the Wii console upright as shown and squeeze gently until you feel a
slight click.
3. The cover can be replaced by reinserting the cover straight down into the console
until it clicks into place.
2. Remove the cover by pulling straight up out of the console.
Horizontal
Position
NOTE: The GameCube Memory Slot Cover can be removed with the same
procedure, and should be removed if you are using GameCube Memory
Cards when the Wii console is placed horizontally.
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