Ok, I think I sorted it out. The function is expecting a char array (C doesn't have strings) but OM was passing a string. I changed the requirement from string to an enumerated type in the C function and passed an integer from OM. At least the there are no errors now but the returned value isn't what I was expecting. At least its a step in the right direction.

I think more examples are needed so if anyone is interested I have attached the files that now actually work.
SteamTables.zip

I compiled the C code in NetBeans as a dynamic library and copied the .so file to /usr/lib/x86_64-linux-gnu, which is one of the standard library locations in linux. The .c and .h files are in the same folder as the .mo file. Using the method where the .h file isn't needed doesn't work for me. I also couldn't get the Python version to work but this does exactly what I want it to.