Creating a TableTop Child Group
Add a TableTop Child group to the Model Tree by dragging the TableTop Design on top of the Table node as shown above.
Note that you now have geometry shown in the Graphics pane. This is because you mixed the Block rule into TableTop. Block is a Design that is rendered in the graphics pane by default; it contains the owner of these parameters, which is a BoxMixin that is (in turn) mixed into the Block Mixin.
Edit Select Child Rules
Edit the Child Rule for height as shown above (Formula is this.thickness).
–This draws from the Parent MyTable load parameter, ensuring that the TableTop is always thick enough to bear the mass (load) placed on it
... since thickness is calculated based on load.
Click on the extents icon: 
Edit the Child Rule formula for length, making it this.length.
Again, click on the Extents icon.
Edit width, making it this.width.
And again, click on the Extents icon.
The resulting UI should look like the following:
