If notepad is already started, you should write:

// import the function in your class
[DllImport ("User32.dll")]
static extern int SetForegroundWindow(IntPtr point);

//...

Process p = Process.GetProcessesByName("notepad").FirstOrDefault();
if( p != null)
{
    IntPtr h = p.MainWindowHandle;
    SetForegroundWindow(h);
    SendKeys.SendWait("k");
}

GetProcessesByName returns an array of processes, so you should get the first one (or find the one you want).

If you want to start notepad and send the key, you should write:

Process p = Process.Start("notepad.exe");
p.WaitForInputIdle();
IntPtr h = p.MainWindowHandle;
SetForegroundWindow(h);
SendKeys.SendWait("k");

The only situation in which the code may not work is when notepad is started as Administrator and your application is not.