I have same issue trying to work on Air Force Officer Performance Report with the XFDL ext. I tired Adobe reader 9 & Word 2011 both gave error message & couldn't open file.
Apple TextEdit was able to open - you could rescue some text if needed but to wade through the lines of code to extract large amount of text or to attempt edit would be a nightmare.
Unfortunately best option I've found thus far is to use emulator like Parallels or VMWare Fusion and run the blasted GOV MIL program shackled to Windows. Grrrr
I do have an old Mac version of IBM Workplace Forms Viewer (2006?) however it will only allow you to view NOT edit or even print or save the forms. If you only want to view that might be an option if the pgm is even available anymore? It was once a free download on a GOV grants site.