I would like to process MODIS L1B data.
For this purpose I downloaded the file:
MOD021KM.A2001145.1050.004.2003082061406.hdf

Together with this file I got the met file:
MOD021KM.A2001145.1050.004.2003082061406.hdf.met

I created a parameter file including the lines:
ifile=dir/MOD021KM.A2001145.1050.004.2003082061406.hdf
met1=dir/MOD021KM.A2001145.1050.004.2003082061406.hdf.met

The program recognizes the MODIS file as a "MODIS Terra Level-1B HDF-EOS product".
However, the "met" results in the error:
****ERROR: get_ancillary: openHDF: Cannot open file -- SDstart failed on dir/MOD021KM.A2001145.1050.004.2003082061406.hdf.met

If I delete the "met1" everything works fine and the program uses the file
"S19461993_COADS_GEOS1.MET_noon"
However I would like the use met data of the time of image recording.

Can someone explain what goes wrong?

The file 'MOD021KM.A2001145.1050.004.2003082061406.hdf'  is not meteorological ancillary data, it is simply an ascii dump of the HDF metadata for the file 'MOD021KM.A2001145.1050.004.2003082061406.hdf', hence the error when you tell msl12 that it is an ancillary data file. 

The file 'MOD021KM.A2001145.1050.004.2003082061406.hdf'  would be equivalent to an OCDP file 'T2001145105000.L1A_LAC' if the OCDP processed MODIS-Terra data.  You could then use the 'Ancillary File Selector' utility located at:

http://seawifs.gsfc.nasa.gov/sdps/cgi/find_ancillary/fan_main.pl

to find the ancillary data necessary to process this file.  Unfortunately, this utility will not work for this case.  You can, however, make an educated guess as the the files needed. 
All necessary MET and OZONE files since 1996 are available via FTP at:

ftp://oceans.gsfc.nasa.gov/METOZ/

The OZONE files are daily files centered on noon GMT.  So if your file is prior to noon GMT (for the file above it is 1050 GMT), you'll need the OZONE files for the day and the previous day  (if the L1B file had a time  greater than noon GMT you'd need the file for the day and following day).

The MET files are usually every 6 hours, so you'd need the two files that bracket your L1B file time (in the case of the file above these would be the 0600 and 1200 hr files)

Here is an example met data input for your par file:

MET1=S200114506_NCEP.MET
MET2=S200114512_NCEP.MET
OZONE1=S200114412_EPTOMS.OZONE
OZONE2=S200114512_EPTOMS.OZONE

Hope this helps,

Sean