WP8 MvvmLight namespace missing and EventToCommand doesn't exist

The reference that you use to include the command is wrong. The correct reference is

xmlns:Command="clr-namespace:GalaSoft.MvvmLight.Command;assembly=GalaSoft.MvvmLight.Platform"

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There's a trick to obtain this reference without writing a single line of code.

After you have downloaded the MvvmLight nuget package, compile your project and then open your xaml file in Expression Blend.

Then click the Assets icon on the left toolbar (the bottom one) and start typing "eventtocommand" (see picture below).

Once you see EventToCommand appear in the Assets panel, drag and drop it on top of your ToggleSwitch. That's it! The reference will be added into your xaml automatically as well as the actual command code.