This was a very old post but it sounds like the original poster was working in a metric drawing and metric profile then accidently started using the imperial profile. This can happen by clicking on the wrong desktop shortcut. Changing the profile will cause the wrong catalogs to be loaded which in turn will cause the problem described by the original poster.
To fix the issue change to a metric profile or make sure to start the program using the correct desktop shortcut.
The behavior that you descibe is 100% identical to the behavior when a part is selected that does not have a catalog associated with it. I.E. having a metric catalog loaded while working with imperial parts. The most common scenario for this happening is selecting the wrong shortcut for starting MEP. By default MEP will install the Imperial, Metric, and Global versions of the program and place shortcuts for each on your desktop. Most of the time people accidently click on one shortcut when they intended to click on another. The obvious solution would be to remove the metric and global shortcuts from your desktop if they exist.
The next step to check if the above did not help would be to just verify that you have the correct catalogs loaded. This can be done by typeing DuctFittingAdd and verify the name of the catalog that shows at the top level of the folder heirarchy. Additionally you can go into the options dialog and select the MEP Catalogs tab and check which catalogs are loaded. Additionaly I would check the Profiles tab and make sure that the imperial profile is the current profile or if you operate under a custom profile that it is setup correctly. Profiles do determine which catalogs get loaded when the program starts up.
Please let us know how you make out solving this issue.