If you already selected the design from the marketplace, (done by clicking the blue button "Free"), you can go to the content manager and click the Design section. There in the left bar, you should see a button called Purchased Designs, from there you need just to activate it.
Everything was ok till (Purchased Designs). I can't find this button :(
If you can't find that button is possible that you have the layers expanded. Try to click "Layers & Explorer" and you should see the general menu of design.