You can also manually invoke "Download Sources" action from Maven popup menu on any jars (including those that are not managed with Maven). Then plugin will use Maven repository indexes to lookup corresponding Maven artifact and download its sources.
Automatic source resolution for Maven dependencies can be turned on from Maven preferences. Go to Window > Preferences... > Maven, then enable "Download Artifact Sources" setting.
In case if sources are not available from Maven repositories, you can manually attach source archive or local folder from the Properties dialog available in popup menu on any jar file.
Dependency Management page also explain how to configure project to enable resolving dependencies from the Eclipse workspace.