Your process was 100% correct.

To burn the CD you now use the "Burn CD" (or similar) command within UltaISO.
eg:
Open the MDF file
Select "Burn CD"

Within UltraISO there will be a CD burner option screen somewhere, and there you select the lowest speed.


Dunno why you saw "unknown file format", try again. You were definitely doing it the right way.

Once you have burnt the CD, you can then re-check the navboot file by opening it from the CD using NavTweak. That will confirm if the CD is correct.