I found the Debug class in [install_dir]\java\jre\lib\security.jar. In order to find out which jar file contained the needed class, I modified the startserver.bat file by adding '-verbose:class' to the Java command line. Additionally, I installed the 4NT utility from JPSOFT.com which allowed me to redirect the console output to a file which I examined to find the following line:
[Loaded com.ibm.misc.BASE64Decoder from .....\security.jar]
Since the com.ibm.misc was the package I was interested in I unziped security.jar to see if it contained the Debug class. It did!