1st , try:
Application.Worksheetfunctions.SumProduct

the:
try to remember under VBA, there is no matricial formula, so you can
not hope [ .Range("OrderMonthName") = monthName(m) ] to be a vector
(because there's no vector in VBA)

BUT you may try

tot = tot + cdbl( [SumProduct((OrderMonthName=B1)*1)] )