I'm having the same problem with my installation. The install was fairly easy and the admin system works perfectly. However, there is not coupon box on the checkout_payment.php page. I'm not getting an error - the option to enter a coupon just doesn't show up.
The only information I've read is that there is some issue with setting a unique value to the module sort order. And I've read a recommendation to remove and then re-install all Order Total Modules. Neither of these recommendations works. But I have noticed an odd issue with my sort order settings. I can edit each module and set the sort order for each and this value shows up on the right where it displays the details under the remove or edit buttons. But the list of modules still shows the original modules listed with their original sort order. The Discount Coupon module sort order is blank in the listing. I assume there is something wrong with the Module install and sort order update but I have no clue what it could be.
Does anyone have any idea what the solution is for this?