The Thematic Change Detection tool is designed to detect change between two classified image. What sort of file do you have? You will need to somehow have ENVI detect it actually is a classification image. If it is something like a TIFF, you could open it and save it to ENVI format in either ENVI or ArcGIS 10. Then use the Edit ENVI Header dialog to change the file type to 'Classification' where you specify the number of classes in the file. Save the header and open it in ArcGIS. It should then be recognized as a classification file. You could probably do the same in ArcGIS.