I think  u do't know about EnsureTemplates without call of this method, we never find any template field  check box  

listWebGrid.DataSource = ((DataTable)ViewState["voidCertDataSet"]);
listWebGrid.EnsureTemplates();

by 

 CheckBox chkbx = row.Items[0].FindControl("IsSelected") as CheckBox;

ok fine for help 

this is solution.