I would think this would work:

tell application "Finder"

set source_file to (choose file with prompt "Choose File...")

end tell

tell application "FileMaker Developer"

tell database "LexiconProduct"

tell layout "Control Panel"

set cell 247 to source_file

end tell

end tell

end tell

It always stops at the cell reference and puts up a Dialog "blah, blah, blah. Object not found"

TIA

Have you tried using the name of the cell in quotes?

And, by the by, Filemaker Developer can be referred to as "Filemaker" in OSX

So why not use:

tell application "FileMaker"

set cell "NameHere" of layout "Control Panel" of database "LexiconProduct" to source_file

end tell

Good luck !