You should not set the property in the "sonar-project.properties" files, because this property may not point to the same folder on different machines.
Instead, you should set this property in the "conf/sonar-runner.properties" file that is located in the %SONAR_RUNNER_HOME% folder where you installed Sonar Runner. It should look like:
sonar.fxcop.installDirectory=C:/FooFolder/FxCopInstallFolder
Note that you must use '/' and not '\'.