From your story it seems like you download the widget from the marketplace outside of the studio pro/studio.
If you open the marketplace from your project in the Studio/Studio pro (click on the shoppingcart icon) you should be able to download the widget directly, and it should appear in your toolbox.
Hope it helps
Move the mpk file to your project_directory/widgets, then in your studio pro just press on F4.
or better to download directly from your studio pro by clicking on the cart icon in the top-left.