OK, that did it. Went through the 'Recharge Diamonds' route again, and found the problem.
For whatever reason, if you select Palenque as the server on the Diamond Recharge page, and choose 'Canada' (where I live) as the language (the default for Palenque is Spanish), the "REDEEM COUPON" button at the bottom does NOT appear. I had to change the language to 'Australia' (under Palenque, for whatever reason, there is no 'United States' option), and sure enough, the option to redeem coupon magically appeared.
All else went perfectly, and I have my dias.
Thanks to all who helped.
Finally, CHEERS!!