Make sure that FxCopCmd is installed and located within your path.
Then:
For a single module install or a multi-module, non-aggregated reports, type:
mvn org.apache.maven.dotnet.plugins:maven-fxcop-plugin:fxcop
For a mult-module, aggregated report, type:
mvn org.apache.maven.dotnet.plugins:maven-fxcop-plugin:aggregate