I think you're pasting it into the wrong place.
Code snippets that start with %%WGT are widgets and should be pasted directly onto a card. Not into a script container or a new prototype, just flat out paste it directly onto any normal card.
After it's in your deck it exists as an available contraption for your project. You can delete that pasted version of the contraption if you want and then make new copies of it from the widget menu (New Contraption...) later.
And of course, you can also freely open it's prototype screen to edit it once it's there.
(Similarly a code snippet that starts with %%CRD is a whole card, and %%IMG is image data)