The best thing to have is a digital certificate, not only to access Suma, but other websites such as Tráfico etc etc.
You can apply for one via the Spanish Treasury website (see link below), but you have to go to a Social Security office or a Tax office to prove your identity. They then send you an email with a link to download your digital certificate. This is then stored on your computer and comes up as an option when you try to get into an area of a website that needs it, such as Suma's "virtual office". I have successfully obtained a certificate, and the process wasn't too difficult! Good luck!
Geoff.
http://www.fnmt.es/en_GB/ceres