We tried using enumeration with one of the properties of an attribute type and received an error message that only attribute types of AutoNumber, Currency, Float, HashString, Integer, Long, and String are allowed. We used enumeration with a different object and it worked out fine.
Have you created a input field in one of your user interface screens to display the enumeration? I think you have used a textbox instead of a dropdown to render the enum. Try deleting the text box and replace it with a dropdown. It is also possible that you defined a search field for the enum, again try replacing it with a dropdown.