Hmm, I went through those steps and all it said to me was “amazon assistant - Your Installation is Complete”. No $5 off offer anywhere I could find, and I was already signed into Amazon and therefore signed into the extension. I may hit up their customer service when I get a chance and find out what the deal is.
(Edit) I contacted their chat support and the rep said “I see that it was not issued, in this case I am manually issuing the promo”. So he tossed me the $5 manually, and it only took about 3 minutes. Easy money, thanks!