If you've downloaded the source of CAS and its by itself should be a mavenized project then, pom.xml should be inside. so you should just 'cd' to the root folder of your source and type
mvn install
or
mvn package
The first command will install your jar into local maven repository, the second one will only prepare the war in the target folder of the module with packaging war in the source.
If you encounter any errors, please post them here.
Hope this helps