During the installation, why is the "Next" button diabled after I entered the serial number?
One reason can be that the serial number you recieved from us doesn't work but that is very unlikely.
You have to make sure that all the registration information you entered (in "User Name", "Organization" and "Serial Number") is exacly the same as the one you recieved from us. Make sure that no leading or trailing space characters are in these fields.