I tried to convert an object, but it gave an "Unrecognized file type" error.
Interchange tries to automatically recognizes the format of a file. It does this through cooperation with all Converters that are installed. If you have deleted any "*.DSC" or "*.DLL" files from the Interchange directory, Interchange will not be able to recognize files correctly.
Some file formats are free-form and difficult to recognize automatically in all cases. You can force Interchange to attempt to recognize the file as a certain format. For more information, see the Options / File recognition section in the preceeding Interchange Window chapter.

I tried to convert an object, but it always gives a "Can't create output file" error.
Interchange needs to be able to create a file on the destination drive and directory. Is it possible that the destination drive is a read-only network drive or a write-protected floppy?

My model file doesn't appear in the file list.
Interchange uses the wildcard to filter the list of files shown in the list box. Change the wildcard to "*.*" to see all files in the directory. An easy way to do this is to select the "All files *.*" entry at the top of the list in the combo box at the bottom of the window.