You need to enter your username and password you normally use when connecting to the MXM Gallery via your web brower, in the preferences of Maxwell Studio and - if you are using a plug-in - also in MXED. Note that the username/password is not the same as for the Customer Portal. Please check first that you can login to the MXM gallery via your web browser: MXM Gallery. The login is in the upper right corner of the web page.
To enter your login information in Maxwell Studio and MXED:
open Maxwell Studio and MXED (found in the Maxwell install folder, or click the MXED icon on your desktop)
Go to File>Preferences>Materials>MXM Gallery and enter your Username and Password