This probably means that your license is already activated on another site and you do not have any additional sites available in your license level.  You can manage the sites associated with your license by logging into your account on our site.  Then, under the section titled YOUR PURCHASE HISTORY you will see a link for VIEW LICENSES.  Click on that link and it will take you to your list of products with licenses.  Click on the MANAGE SITES link – there you can remove an existing site.  After that your activate button should work to enable your new site.