elton,
I believe your issue is with the URL.
You posted:
products_Search.php?InProcess=AddToCoupon&CouponID=3
The page you need is:
Products_Search.php?InProcess=AddToCoupon&CouponID=3
Capital "P" for "Products_Search". In Dreamweaver open up the referring page and do a find and replace, find products_Search and replace with Products_Search.