We try not to support authenticated pages ( pages which require username and password for log in), because of privacy concerns. Co-browsing works because part of the website is downloaded on Dimdim servers. This content is deleted at the end of meetings, but it still raises privacy and legal concerns in case of authenticated content. Dimdim web page sharing feature currently does not support pages which require authentication due to security and privacy concerns. 

We recommend using desktop sharing in this case. You can start desktop sharing, minimize all applications, open a new browser and use the site you want to present.