In order to login to Sniffie Service, you need to have received a registration email and clicked the registration link to activate your account. In the registration email, you have received a one-time password for your first login. When you first login, the service prompts you to change your password.
If your login is unsuccessful, the login page gives you a reason as to why that is.
Only one active session (one for Sniffie extension and one for Sniffie web service) can be enabled at any given time. Thus, when you login to Sniffie service from another browser, your previous session will expire.