With composite datawindows, you can't use computed fields. The computed field menu-items and toolbar buttons are disabled. However, you are able to use 'standard computed fields' such as 'todays date' or 'current page of'. So, if you want to create a computed field of your own, first create a standard computed field i.e. todays date. Then, just doubleclick on it and voila, you can enter your own computed field expression. If this is a bug, I'm not sure.