If you have reached the license activation limit there is an option to deactivate any site that you do not want the plugin registered to, in this way it will open up a license that you can use to register a new site. To register a new site , please add the activation key to the plugin on the new site.
Please login to your account on http://www.wpbackitup.com/ then go to the account section of WPBackItUp. This is what you should see on your screen:
Click the view licenses link on the top right there should be a manage sites link, click that and it will show all the sites that are already registered and there is an option to Deactivate site.