How to Schema Diff two databases using MySQL Workbench?

If you don't see this option directly you have to:

1. Click File and New model
2. On the new model window which has been opened, click Database and Compare schemas.

you can open "Model view"

and choose "Compare schemas" option

The database diff tool is part of the MySQL Utilities that you can access through MySQL Workbench in Plugins -> Start Shell for MySQL Utilities. You can find the relevant documentation here.

Just go to File->New Model, then Database -> Synchronize with any source. You have to be in model view to see this option in the Database menu.