Using the same method that you used in creating the TableTop, create two more Designs, which will be used in conjunction with your Table Design:
•LegPair
•Leg
Include the Block Design as a Mixin for both.
Resulting screen when you select Leg:
Above, a mistake: note under Mixins in the Context pane that the Block Mixin is not present in the Leg Design.
In a case like this, simply click on the + icon above the Mixins pane. A dialog pops up, "Add Mixin(s) to Design"; in that pop-up, select Block and then click OK. (If you made a further mistake by, say, selecting the wrong Mixin, you can click on Revert before doing the next step.
Click Apply before moving to another Design or Rule.
Result:
