Seems to be applicable to docs preceding 2.4.1.
http://hadoop.apache.org/docs/r2.4.1/hadoop-yarn/hadoop-yarn-site/CapacityScheduler.html, however, doesn't have the installation section.
In 1.0.4 it says:
You can also build the Scheduler from source by executing ant package, in which case it would be available under build/contrib/capacity-scheduler.
To run the CapacityScheduler in your Hadoop installation, you need to put it on the CLASSPATH. The easiest way is to copy the hadoop-capacity-scheduler-*.jar from to HADOOP_HOME/lib. Alternatively, you can modify HADOOP_CLASSPATH to include this jar, in conf/hadoop-env.sh.
For ant, it would be under /build/* for Hadoop it would be in ~/lib.
Closing this for now.
Please reopen if you have questions.