Just so anyone else that has this problem won't pull their hair out trying to figure out what is causing it. I just updated this plug in and it broke importing FreeHand MX files into Illustrator CS5.1.0. Importing a FH11 file will open a blank document. I've sent a report to LinoType with the problem.
FontExplorerX Pro 3.1.1 - Build (6543)
File Name: FontExplorer X for Illustrator CS5.aip
version 3.5.0, by HLW
Mac OS - 10.6.8
2.7 GHz Intel Core i5