I think you are missing an equal sign for the .FormulaR1C1 here is one Sub that should work:

Sub Macro12()
'
' Macro12 Macro
'
Dim i As Integer

For i = 1 To 64
    Cells(i, 1).FormulaR1C1 = "=<Element Index=""[" & (i + 1) &"]"" Value=""0""/>"
Next i

End Sub

As @VBasic2008 pointed out, this should give you <Element Index="[2]" Value="0"/> in A1.