Creating an EER diagram from an existing schema is called reverse engineering. In MySQL Workbench you simply start from the homescreen and open menu -> Database -> Reverse Engineer...
This will ask you for a connection to a server and, once connnected, will allow you to select all schemas on that server you want to create a representation from. Simply follow the wizard further until it is done. Provided there are no errors you should get a new model with all the selected content in it.