The dataItem is available only for k-on-change events. However, for expand you can easily get it from the kendoEvent. Note that there is a distinction between selected item and expanded item -- a tree allows you to select an item, and expand a different item. Hope these samples clarify:
1. for Tree: http://dojo.telerik.com/@mishoo/udidI/2
2. for Grid: http://dojo.telerik.com/@mishoo/okEQe
I included the grid sample because it's not really clear to me if you're using a tree or a grid. But in any case, getting at the expanded element is similar for both: call widget.dataItem(domNode), where domNode is the expanded element which you get via kendoEvent. For tree that's ev.node, and for grid it's ev.masterRow.
Hope this helps.
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