Sorry for the late reply. I think it's a permission issue that I should have addressed in the initial post. You need to make sure that you can run sudo renice without a password:
Edit your sudoers file, using sudo visudo in Terminal, so renice doesn't need a password.
The line to add will be:
your_short_username ALL=NOPASSWD: /user/bin/renice
It's vi, so i to enter interactive mode, add the line, then hit the Esc key and type :wq and hit Return to write-and-quit.
(Copied from the KM forum post about something similar: How to run sudo command in KM? - #6 by Nige_S