When constructing a "Send Apple Event" script, you will sometimes need to specify an application to send the event to. When FileMaker Pro shows you a dialog box to select an application, it only shows you those applications that can accept Apple Events.
Issue
After you select the proper application, the name of that application should appear to the right of the words "Target Application". However, if the Target Application name appears as "<unknown>" even after you select an application, the desktop file may be damaged.
A Solution
Restart the Mac, holding control and command down until you are prompted about rebuilding the desktop. After rebuilding the desktop, try to specify the same application. After specifying the application, it's name should appear as the "Target Application".