To add a custom attribute type to the selected model element type, simply click the Add... button on the element's Custom Attribute Types page.
This dialog contains a list of the custom attributes types that have been previously created for other model element types but are not included as custom attributes types for the current model element type. To include one or more custom attribute type(s) to the current model element, select the desired item(s) and click OK. You can use Ctrl and Shift keys to select multiple types The selected item(s) will then be listed among the Custom Attribute Types of the current model element type.
Note that adding custom attributes within a model links the same attribute type to multiple element types, so changing the custom attribute type for some element type reflects the change also to other element types using the same custom attribute type. However, when linking a custom attribute type from a base model to an element type in a child model, the custom attribute type is copied to the child model, so the base model is not affected in the case the custom attribute type is modified in the child model.
Add Custom Attribute Type dialog