The Jetty module will be a SAR - assuming that you want the JBossweb-Jetty bundle and not the JBossweb-Tomcat bundle. If you are building from source, the Jetty SAR will be jetty/output/lib/jetty-plugin.sar. You would unpack that into JBOSS_HOME/server/instance/deploy/jbossweb-jetty.sar where instance is your designated run-time. Normally this would be default. You would then remove the jbossweb-tomcat41.sar directory from the deploy directory.
Otherwise you can download the pre-built JBoss-Jetty bundle from SourceForge.
There is a new Jetty 4.2.14 release but the source build is a bit broken - you need to do a few hacks to get it to build. There is also a pre-built set. You need to get it from the Jetty SourceForge site. I'll write up the build procedure later this week.
Hope that helps.