Assuming..
txtOperator.Text = "Alvin Gutierrez"
txtDate.Text = "9/18/12"
txtMC.Text = "M4"
"SELECT SUM(Mach_Hours) AS TotalHours from myTable WHERE Operator= '" & txtOperator.Text & "' AND prod_date = #" & txtDate.Text & "# AND MC='" & txtMC.Text & "'"
I got your files, may babaguhin ako and idadagdag para meron ka pong reference...
Error tong query mo brad once na na icompile to sa palagay ko, nag aggregate ka hindi ka nag group by