Just tried both versions (uc_coupon_1.5.zip (above) and uc_coupon-5.x-1.5.tar (from drupal.org)) and wanted to say that the one on drupal.org is the old one. The one you need to use is uc_coupon_1.5.zip.
It's pretty darn confusing to have contributions listed like this and I vote strongly for this to change. I believe contributions should be posted on drupal.org from the start so we can use the invaluable issue queue.
I think this module supersedes the one on drupal.org (please correct me if I'm wrong) so I think that this module should replace the one on drupal.org.