ManagedBean is not provided by seam, there is a download link for it in the article.
So does this tag:
belong in components.xml or other xml file?
It belongs in components.xml. You just have to add the appropriate namespace declaration and make sure ManagedBean.java is sitting in a package with @Namepace(
whaever) in the package-info.java file. Have a read about @Namespace in the seam docs if you need more info.