Is the extension showing up in Extension Manager? If it is, then you should open up Dreamweaver and go to the File->Site Import... and this is where you should access Site Import from. Then the WebAssist Site Tools floater will open. Select Site Import from the drop down menu and hit the GO button. This will pop up the Site Import Wizard and this is where you will have to input the serial number. The easiest way to do this is to copy the serial number using ctrl+c and pasting it into the first text input for the serial number using ctrl+v.