If you look at the top of the roll20 page and see the button marketplace. Hover over it and you will have a drop down menu. At the bottom of that list, it should show "my library". If you click on that, you will go to your library and on the right top side is three buttons "your library", "your purchases", "upload". Clicking on your purchases will take you to a page that has all your purchases and you can click one of your purchased packages it will take you to that marketplace page and you can download it from there, if it is allowed. Or you can skip all that and just click on the marketplace button and do a quick search with the package name or artist (if you know it) to find it then download it.
