Do you mean this code?
Code:
<Trigger Property="MyBooleanValue" Value="true">
<Setter Property="Opacity" Value="0.5" />
</Trigger>
Where did you place this XAML fragment to? As the text explains, you need to place this within the Triggers property of a control that has a property named MyBooleanValue.
Use the code segment that follows for a concrete sample.