The popup wasn't loading for me after clicking on the extension in Chrome. It kept showing a blank window. I did some searching and found this workaround:
1. Right-click the extension icon.
2. Click “Inspect popup”.
3. In the window that appears, go to the “Console” tab.
4. Copy and paste this text, then press Enter: document.body.style.height = document.getElementById('UBPv2MainPanelFrame').style.height = '100%';
Frustratingly, the fix is temporary and disappears when you close the Amazon extension. But after doing the above steps once, instead of pasting the text from Step 4, you can just press the up arrow on your keyboard (then Enter), which makes the whole thing a bit easier.
After doing this, it worked and I was able to go through the tutorial, which is within the actual popup, and add the coupon. Good luck!