Copy the code starting at the "%%WGT" part and ending with the last "}" to your clipboard, open Decker, and then choose "Edit -> Paste Widgets" (or just "Edit -> Paste" in web-decker) from the main menu.
The code blocks on this page are what it looks like when you copy and paste one or more widgets in Decker. When you copy a Contraption to the clipboard, it carries along the associated Prototype (contraption definition), which makes it possible to share them like this or easily copy and paste them between decks. The same idea applies to copying and pasting cards.
Contraptions were added in v1.12, so if you downloaded an older version of Decker, please make sure you upgrade!
Does that clear things up?