The MySQL User Manager is an easy to use tool. Simply double click the User Manager node in the Enterprise Manager and the User Manager will open - alternatively, use the context menu Administration & Maintenance | User Manager.
You should have registered the server with a valid root password in order to create users.
The MySQL User Manager
Selecting a user and using the Edit User button will open the User Properties dialog.
The User Properties Dialog
If you create a new user, this dialog will appear as well - but empty of course. If you're modifying a user, and you don't want to modify the password - be sure to NOT change the current value in the Password and Confirm edit boxes.
The bottom part of the dialog has two tabs, one for the server global privileges and once for the privileges on a per database basis.
Database Specific Privileges of a user
By using the buttons on the right, you can set specific properties for a database for the user, you can then select or deselect the privileges you want for this user under the grid.
Exiting this dialog by using the OK button will save the user information, using Cancel will, naturally, cancel all changes.
For information about the different options, please see the MySQL documentation.