You can add model elements to models using the Model
Explorer.
Models comprise one or more model elements. The Model
Explorer view lists the model elements that make up your models.
When you add model elements in the Model Explorer view,
they become part of your model, but are not represented in any diagram.
To
add model elements:
Right-click a model or a model element; then click Add UML > Class.
Type a name for the new element and press Enter.
Optional: Click and drag the new element
to the modeling diagram editor. The element now belongs to your model
and is also represented in a diagram.
Note: If you have set preferences to filter
or to hide model elements, the Model Explorer does
not display the new elements you add.