I see this is several days old, so I'm not sure if you figured it out already. I had an issue when I tried to sign up for Diigo Educator too. My district has recently switched to a .net address instead of our old, cumbersome, .k12.ca.us address. When I initially tried with the .net, I got the "have to verify email address", but when I switched it to my old address (which still works), it went right through.