Two related answers:
Generally, you can edit data in the "Result Grid" in the SQL Editor. From the Schemas Navigator (left pane), right-click on a table and select "SELECT Rows - Limit 1000". This opens a "Result Grid" tab in the SQL Editor. Simply click the cells and add data. On the right you'll see that "Result Grid" is selected by default, and you can change it to "Form Editor" for another input method. Workbench 6.2 simplifies this by adding icons next to each table in the object navigator, including one to select all rows.
For Models, double click on a table (or, right-click on a table and choose "Edit Table"), then go to the "Inserts" tab (you'll see it near the bottom). This field editor is similar to the results grid editor mentioned above. After making your additions (inserting rows), click the checkbox next to "Apply". These entries will be added as a transaction.