Sub MacroTest2()
    With Worksheets("List1")
        ro = Range("d2").Value
        col = Range("c2").Value
        .Range("C6").Formula = "=$" & ColumnLetter(CLng(col)) & ro
    End With
End Sub

Function ColumnLetter(ColumnNumber As Long) As String
    Dim n As Long, c As Byte, s As String
    n = ColumnNumber
    Do
        c = ((n - 1) Mod 26)
        s = Chr(c + 65) & s
        n = (n - c) \ 26
    Loop While n > 0
    ColumnLetter = s
End Function