It should just happen anyway, try it on the showcase here:
is your extendedDataTable in a h:form? it'll need to be because the selection will be done via ajax. When you select a row then
in the chrome browser (for example) you can right click on the table row and select 'inspect element'. The elements tab in the
developer tools window should appear and if you move up from the selected element (should be a div) to the nearest tr it should
have the rf-edt-r-sel class.
It always help to post some code up otherwise we're all flying blind.