Questo si compila correttamente:
<Window.Resources>
<DataTemplate x:Key="MyTemplate1" DataType="{x:Type local:MyClass}">
<StackPanel Orientation="Horizontal">
<Label Content="{Binding Path=Field1}" Width="60" />
<Label Content="{Binding Path=Field2}" Width="60" />
</StackPanel>
</DataTemplate>
</Window.Resources>
e nel punto dove lo usi devi scrivere:
<ListBox Height="300" ItemTemplate="{StaticResource ResourceKey=MyTemplate1}" Width="500" />