Shredder requires an .exe-file to recognize the engine as UCI (it recognize it as Winboard with the .bat-file but can not play games due to weird behavior) and Chessbase requires an .exe-file to even try install Mediocre.
The solution is to use a small program called jlaunch by Manfred Rosenboom.
For Mediocre you simply place the program in the main directory (where the .bat-files are located) and create a file called jlaunch.properties and place it in the same directory.
The properties file should look like this:
mainClassName=MediocreYou need to have paths set to the Java bin-directory on your system for this to work (try typing 'java' in a console window, if you do not get an error you have it set).
Now when installing Mediocre in Shredder Classic or Chessbase you simply use the jlaunch.exe as engine file and it should work.
This works in Arena as well.
Now I just need to track down the problems wbec-ridderkerk had with running Mediocre. That has nothing to do with the above solution since they run it as a winboard engine. I suspect it has something to do with a preset opening book.