This dialog window, which may pop up when you start PowerPoint with think-cell installed, indicates that you are not directly connected to the Internet. To enable automatic update and online quality assurance features in think-cell (all based on secure connections, of course), please enter the username and password to access the Internet through your company's proxy server. This is often known as the Internet password which you also need to access any public website outside your company.
If you check Remember user name/password for next time, the password is encrypted and stored locally on your machine, so that you do not have to enter it each time.
If you check Do not ask for user name/password, think-cell will not ask again. To check for an update, go to More → Check for Update… and enter you user name/password when prompted.