Apparently it cannot just be copy-pasted into Edit HTML section as-is... :-(
But it can be done via the Layouts / Page Elements section! Here is how:
After clicking Add a Page Element, a new window will appear. In the new window, choose:
Well, the layout of these instructions is now all messy... I don't really know how to arrange pictures in a blogger post yet. But hey! after you add the new HTML/JavaScript element, it will ask you for the title (anything) and for the actual code (which you have from whereever you got it). Save, and then you can already arrange your new widget by drag-and-dropping it to the right location:
There you should have it!
:-)
