On my computer, the offer is still a large banner at the top of the Kindle bookstore page. So, this morning I clicked it again to look. It doesn't say anywhere that I have already taken advantage of the offer, but the terms do say it is limited to one offer per account.
Farther down in the fine print of the terms is this bullet: "Within 4 days of activating the offer and spending the specified amount on Kindle eBooks, you will receive an e-mail from Amazon that indicates that the promotional credit has been applied to your customer account. The e-mail will also provide instructions on how to redeem the promotional credit. You can view your available eBook credit balance on any Kindle eBook detail page."
I thought I would note the receiving a confirmation e-mail "within 4 days". I guess Cagnes can wait a day or two more to see if the confirmation and credit balance show up. That seems odd to me that it isn't more quickly automatic. Of course, this offer also said to click the "activate" button for it to work, but there never was a specific button this time. It activated for me originally by clicking the "get offer" button in the banner just to read the terms.