MySQL Workbench charset
I think OP was asking about charset that Workbench uses in its editor and how to setup Workbench to use UTF-8 in GUI - not how to setup default charset used for database table in Workbench. At the moment in Workbench one can set database table charset but regardless of it Workbench will in it's GUI represent data using Latin1 charset!!! Also inserting data into UTF-8 tables through Workbench will result with wrong chars in database. I don't know either how to make Workbench deal with UTF-8 in its GUI.
In "Model Overview", under "Physical Schemata", right-click the database and select "Edit Schema...". Define the character set to the "Collation"-field. (MySQL Workbench 5.2.35)
For further reading, another fix would be to change the display font. Go to Edit -> Preferences -> Appearance. Change the Editor font. It could be that the default font does not display utf-8 correctly, but others do.
Oh, and if you are having trouble changing your font you can try here