probably cause you're not adding the IP into the range fields. You're adding it in the top text box aren't you? Well, that box is only for resolving the IP to a host or to resolve a host into an IP which will be put into those 2 range text boxes with 'start' and 'stop' in front of them.
So load up SuperScan. Enter http://www.yahoo.com
into the top text box. Click the "Lookup" button. Now click the "Start" button.
If you STILL don't get an open port on 80, then maybe you didn't tell it to scan that port.
Well, if you can't get it to work this time, tough. I've lost all patience.