To get more info, you need to look into the help using MS Help Viewer. The event arguments have just one field, SelectedItem, which returns the currently selected list item (see the iGSelectedItemChangedEventArgs type).
It seems this event does not require any other additional info - row and column indices aren't applicable here. Most likely, you are trying to use the SelectedItemChanged event for a purpose it is not intended for. For instance, if you need to know whether the value of a cell with an attached combo list has been changed, you should use the AfterCommitEdit event which sure provides you with the row/column params.