If your MODTRAN path contains spaces (common on Windows where MODTRAN might be installed in "C:\Program Files"), then DIRSIG cannot launch MODTRAN from either a
"Classic" (make_adb) or "Threshold" atmosphere configuration.
The problem is that QProcess::execute() interpretes a path that contains spaces as the program name followed by arguments. So it attempts to run the first "token" of the path (the part before the first space) as the program and the remaining part as arguments. Hence a path like "C:\Program Files\Modtran522\modtran.exe" results in DIRSIG (via QProcess) trying to run a program called "C:\Program" with the argument "Files\Modtran522\modtran.exe".
The solution was to simply wrap the path with double-quotes.