While we don't provide out-of-the box support for UML class diagrams, you can create a reasonable representation of a UML class using the table shape from the Basic shapes library.
The table shape requires some customization to make it look like a UML class, so we've provided a custom shape library attached to this article. To use it, first download the file and place it in a folder of your choice. Then open a diagram in CaseComplete and click the Open Shape Library button found near the top of the Diagram Tools panel on the left side of the diagram (look for the buttons that have a green tinge to them) and open the file you just downloaded.
The shape library also includes an arrow for the generalization (inheritance) relationship as shown above. You can also create many other endpoint styles by placing a regular association and then clicking the Arrowheads button. You can find the Association button and the Arrowheads button in the Drawing Tools ribbon tab along the top.
As an alternative to drawing classes in CaseComplete, you can also export to many UML drawing tools. Your definitions and their fields will be exported as classes and class attributes.