If the reference listed in the Available references list is preceded with "MISSING:" clear the check box to remove the reference ...

If the reference listed in the Available references list is preceded with "MISSING:" clear the check box to remove the reference if it is no longer required.  If you still need to use the reference, clear the check box entry for "MISSING: " in the Available references list, and then create a new reference to the file using the Browse... button.  If this is an installed database application, you may need to reinstall or repair the application.