If you use Eclipse IDE for your Java development, it can be configured to launch CayenneModeler when clicking on "cayenne.xml" file in the project tree. Here is the instructions for Windows, other platforms should work in a similar fashion.
In Eclipse Properties panel go to "Workbench -> File Associations". Add an association for "cayenne.xml" to open with "external editor". Select "modeler.bat" file as an external editor. Caveat - CAYENNE_HOME variable must be either defined in the environment, or you may need to edit modeler.bat and set it right there, otherwise modeler will not start.