MySQL Workbench, how to refresh EER diagram?
From the MySQL Workbench 5.2 window, select Database -> Synchronize model which will allow you to synchronize the model (EER diagram) with the database (you need to create a saved connection) for it. 

I recommend this method over just reverse engineering the model from the database since it maintains the layout of the EER diagram. One caveat is that you need to add new tables to the EER diagram manually.