Start by locating the dsmlink.prefs file. This file can be found in the DSMLink main program directory. Unless you specified something different at install time, this would normally be:
C:\Program Files\DSMLink
Edit the file with Notepad or some other basic text editor. Be sure that DSMLink is not running while you edit this file. Once you save the file, run DSMLink again to see the effects.
Add the following line to your dsmlink.prefs file. You can add this line anywhere in the file.
debug=0xFF
Save the file and run DSMLink again to reproduce whatever problem you were having. When you have reproduced this problem, exit the DSMLink application and go back into the DSMLink main program directory (the same directory where the dsmlink.prefs file was located). There should now be a debug.txt file in that directory with the new debug text in it.