x:Bind
doesn't support design-time data yet. Maybe it never will given it's designed for compile-time bindings to boost performance (I wish it would though).
For simple UI testing purposes, I would add a FallbackValue
to the binding expression to force the designer to show me some dummy text. However, don't forget to remove it once you are done with the design.
<TextBlock Text="{x:Bind MyMoney, FallbackValue='$10,000,000'}" />
Update
Now it's easier with the new design-time data support.
<TextBlock Text="{x:Bind MyMoney}" d:Text="$10,000,000" />
Read more from here.