How to open the wifi sign-in page when it doesn't pop up automatically
When the log in gets eaten by other software or times out, just open your chosen web browser and navigate to http://captive.apple.com

Some interesting tidbits: 


How captive.apple.com is used by non-Apple devices?
How to disable captive.apple.com?
A clear question and answer on implementing a captive portal with key triggers documented for multiple vendors and versions of OS