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So, for the Community edition, does it mean that we cannot view the master data without logging in as either "admin" or "user"?
You can customize login-config.xml to add a new user (see http://www.talendforge.org/forum/viewtopic.php?id=22539), but you won't be able to set particular roles on the master data (set read-only privileges to a user, for instance), using Talend Open Studio for MDM.
This is where Talend Enterprise MDM brings value, enabling to create roles, and set a granular security.
PS : The built-in users for the Web UI are "administrator/administrator" and "user/user". There's also a technical super-user, that can only connect through the Studio, whose login is "admin/talend".