To model a datatype property as a class attribute

1. Create a class, name it Class 1.
2. Click on the black circle with the white plus sign in the center of it.
3. Select Property.
   
4. A default property appears in the class. The property is also called a class attribute.
   
5. Double - click on the default property. Its Specification window appears.
   
6. Click on the Type field. A shortcut menu appears. Select boolean. (or any datatype property you want)
7. Click Close.

The result should look like this:

The class element with a datatype property unnamed1.