Sorry, this forum space isn't for EM Express or 12c-specific questions, it's for the Oracle SQL Developer IDE/GUI
Thanks!, let me go to the correct forum
Repost in the Multitenant forum and see my reply dated Apr 22, 2014 3:54 PM in this thread:
Oracle 12c uses a NEW, and different, multitenant architecture. That requires that certain, basic operations (e.g. creating users, connecting) are done DIFFERENTLY.
If you want to avoid the RTFM responses that might be coming your way read jump the gun on them by reading chapters 17 and 18 in the Database Concepts doc. Those chapters, and my reply in that thread, will answer your question.