I'm not familiar with VBA specifics, but this should get you started.
Code:
' Local variables
Dim sum As Double, prod As Double
Dim i As Long, j As Long

sum = 0 ' Initial value for Sum
For j = 1 To n ' Calculate Sum
  prod = 1 ' Initial value for Product
  For i = j To n ' Calculate Product
    prod = prod / (1 + y(i))
  Next i
  sum = sum + prod
Next j
r = (1 - y(n) * sum) ^ (-1 / n) - 1