expandItem

Method which expands the table row with index passed as an argument. If the index corresponds to nested table item, all parent items will be expanded to top.

expandItem(index)
index	Integer	The row corresponding to the index will be expanded.

Example:

function ExpandFirstItem() {
    var masterTable = $find("<%= RadGrid1.ClientID %>").get_masterTableView();
    masterTable.expandItem(masterTable.get_dataItems()[0].get_element());
}