I think this will work:
=IF(AND(C82>0, C82<13), SUM(Forecast!C53:Forecast!OFFSET(C53,0,C82-1)),"")
As an example:
If C82 contained the value 4, then
OFFSET(C53,0,C82-1) would evaluate to
OFFSET(C53,0,4-1) which would evaluate to
OFFSET(C53,0,3)
OFFSET(C53,0,3) evaluates to (C53 + 3 columns)...