Unless something has changed and I am unaware of it, the Format Converter does not work with MIDI files.
The method in the link you provided has not changed and it still should work in Win 5.581. Here is another tutorial that has the same method, but with slightly different instructions...
The red notes (Note:
) from that tutorial are areas where people easily get stuck on. I have a feeling you may have missed out on one of them. Which is one of easy mistakes to make during the MIDI to WAV conversion.
The DiskWriter method provided by Koopa is the same idea, but has a different functionality. It may not work in all cases.