This may help you, if you're trying to do what I think you are.
hWnd being the handle to your edit control and lpString being the text you want to send there
void AppendWindowText(HWND hWnd, const char * lpString)
int iLength = GetWindowTextLength(hWnd);
SendMessage(hWnd, EM_SETSEL, iLength, iLength);
SendMessage(hWnd, EM_REPLACESEL, 0, (LPARAM) lpString);
SendMessage(hWnd, WM_VSCROLL, SB_BOTTOM, (LPARAM)NULL);
I use that code in a chat client I'm working on, just call that function when you need to add text to the edit control