What do you get when you choose Select Dictionairies? Are you able to select different dictionaries?
You might also check the Applications folder for FileMaker and see if the dictionary file exists. You appear to be a Mac user going by your screen shot and I'm on windows, but there should be a dictionary file inside the dictionary folder. On windows, it's found in the extensions folder.
And as a Mac user, you might try trashing the FileMaker preferences file.