www.wysiwygwebbuilder.com

Even with deactivated Firewall, I cannot install Extensions, as my Serial number is not found.

Your account seems to be ok, I was able to login with your serial number.
http://extensions.wysiwygwebbuilder.com/

Please disable your firewall or any other utility which might prevent you from logging in.
Or try to login on another computer or browser, so you can be sure it's not just this PC that is causing the problem.

Also note that you can download the extensions directly from within the program by using the Extension Manager.

Thanks, made it work now!!!