Code Runner does not run C# code
Have you installed Mono and added the configuration to CodeRunner to enable C# support?

I found a solution if you have installed Mono over at MacRumors forums posted by user David Sheeks outlined below:

Configure CodeRunner

Open the CodeRunner Preferences dialog, go to the "Languages" tab, click the "+" icon and and specify "C#" as the Language name.

On the right you have fields where you can populate the language settings. Check the "Language uses compilation script" check box, enter /usr/bin/mono $compiler for the Run Command and set the File Extension to cs

Click the "Edit Script..." button and append the following to the compile.sh script (this will open in the CodeRunner editor):

file=$1
/usr/bin/mcs "$file"
status=$?
if [ $status -ne 0 ]
then
exit $status
fi
echo $file | sed -e "s/\.cs/.exe/"
exit 0


Mono installs it's compiler and runtime executable in /usr/bin and this is verified with the current version of the Mono SDK.

If you have done part of the above, and not setup the path's correctly, I would assume that the missing path it's complaining about is the fact that you either a) haven't installed Mono and the config can't find the compiler or b) youve installed Mono and the config in CodeRunner is invalid.