debug messages, how to turn them on?

[Stuart Axon <appg18@dsl.pipex.com>]

I see debug messages in the code like; (in mfs.c)
Debug0((dbg_fd, "selecting drive fn=%x sda_cds=%p\n",
	  fn, (void *) sda_cds));
and tried
c:\>  dosdbg #dMRWgnsEPSXchkpxCIdimrv
Current debug message class settings:
#  interrupt   ON          A  unknown     OFF         C  cdrom       ON
D  dos         OFF         E  EMS         ON          I  IPC         ON
M  DPMI        ON          P  pktdrv      ON          Q  unknown     OFF
R  disk Reads  ON          S  unknown     ON          T  unknown     OFF
W  disk Writes ON          X  X support   ON          Z  unknown     OFF
a  unknown     OFF         c  config      ON          d  disk        ON
g  general     ON          h  hardware    ON          i  port I/O    ON
j  unknown     OFF         k  keyboard    ON          m  mouse       ON
n  network     ON          p  printer     ON          r  PIC         ON
s  serial      ON          u  unknown     OFF         v  video       ON
w  warnings    ON          x  XMS        
ON                                   

...and don't get any debug messages anywhere; I would expect them to be
on stderr?  I tried running dosdebug and nothing comes out on that... is
there a config option I am missing?