SendMessage to another Application

[Xocitus]

            string lpszParentClass = "SciCalc";
            string lpszParentWindow = "Rekenmachine"; //Dutch translation Calculator
            string lpszClass = "Button";

            IntPtr hwnd = FindWindow(lpszParentClass, lpszParentWindow);

            IntPtr childhwnd = FindWindowEx(hwnd, IntPtr.Zero, lpszClass, "5");

            SendMessage(childhwnd, (int)VKeys.VK_2, 0, 0); //VKeys.VK_2 = 0x32
            SendMessage(childhwnd, (int)VKeys.VK_2, 0, 0);

Supposed to press the button five on MS Calculator.
It doesn't give any errors but it just doesnt work.
What did I do wrong?