How to connect to a Wi-Fi network that requires a special sign-in
I have had this problem many times with my Mac at places like hotels and universities that use a web based login.

smoosher is on the right track. After the WiFi login, you need to go to a web site that does not use HTTPS. The New York Times is an example (http://nytimes.com). This will cause the redirect you need to the second login page. Sites that use or offer HTTPs probably won't work for the redirect.