Connecting to Captive portals

I have a Mac OS X version 10.7.5. At the university I attend in order to connect to the Wifi, you are redirected to a sign-in screen that requires your student ID and password. I am usually able to connect to the school network, but for some reason I am no longer able to connect. The redirect page does not pop-up on any of the browsers that I have tried (Safari, Google Chrome, and Firefox). I have tried turning the wifi on and off, shutting down and restarting my Mac, and resetting the browsers, nothing has worked. However, I can connect to the Wifi at my house (it does not require a redirect) so I do not think that the problem lies with the wifi connecting ability of my computer. I was wondering if anyone else has had this problem and if so now did you resolve the problem. I am a fairly new Mac user ( only about a year), so I might require step by step instructions. Thank you for your time and consideration to this issue.

You really need to ask the IT folks at your school for help with this. We have no idea how your school network operates. I can only guess that it requires a proxy setting on your Mac in order to connect. You would do that from System Preferences, Network, Advanced, the Proxy tab. But since there are many choices of the type to use, you need to ask them.