Download the certificate from here: http://wifi.brown.edu/rootcas_311_6215.p7b
Once downloaded, install it on the phone.
Once the certificate is downloaded you can connect to Brown wireless. Click on the Brown wireless network and add your Brown username credentials and turn on Validate Server Certifcate.
Follow additional Instructions
The wifi.brown.edu already has the instructions availabe. You can click on "load XpressConnect manually." Click on "Other Operating Systems" and then click on "Windows Phone 7 & 8".
If you're already on a Windows phone, you can click this link to go there now: http://wifi.brown.edu/generic_windowsphone78.html
The certificate file is also available on this page, as well.