Set Instance Classifier - List comes from...?

[DMT]

I want to start putting together my UI prototype in the form of object instances in a "User Interface Object Model" view. I want to do this knowing when I do the detailed design of the UI classes, I can come back to the objects and set their associated classes from which they're instantiated.

When I right-click the object and click "Set Instance Classifier," however, it just shows two entries--<none> and the name of the object I just created. It doesn't show the test class I created in another package.

I also saw this issue when I was creating a sequence diagram using boundary and entity objects; the instance classifier dialog only showed me the objects in that sequence diagram.

Anyone have any idea on how I can associate a class with an object in one of these two locations after the fact? I'd hate to find out that I can't do it, because that will cost me a lot of time right now trying to figure out the actual classes for my UI instead of letting me focus on assembling the mock-ups.