Hi, I was also having this problem.
On the coupons_Update.php page I got error 404 when I clicked on the edit products button. The page is looking for the URL products_Search.php?InProcess=AddToCoupon&CouponID=9
To fix this error edit coupons_Update.php and search for products_Search and replace with Products_Search. (capital P for products). Save the file, upload to server and you should be sweet.
EDIT: Apply same fix to coupons_Detail.php as it is incorrect there as well.