Thanks for your response to this issue. The code you provided gets me to the <TD> tag that holds the CheckBox. I used the following code to get to the actual Checkbox in the <TD>:
function OP_RadGrid_RowClick(sender, args)
{
var oMasterTable = m_oRadGrid.get_masterTableView();
var oRow = oMasterTable.get_dataItems()[args.get_itemIndexHierarchical()];
var oCell = oMasterTable.getCellByColumnUniqueName(oRow, "CheckboxSelectColumn");