The project I'm working on uses a function that appends text to a text box. Might as well paste the code.
Code:
// textbuff is a global var containing the entire text of an edit box
// it should be declared like:
// char textbuff[1024];
// 1024 is how long you want to let the buffer get.
// This example uses information to get the HWND from a dialog
// I used resources, so IDE_TEXT_BUFFER is a #define for the
// text box I made in the resource editor
void AppendText(char *newtext)
{
strcat(textbuff, "\r\n");
strcat(textbuff, newtext);
SendDlgItemMessage(
hDlgChat, IDE_TEXT_BUFFER, EM_SETSEL, 0, -1);
SendDlgItemMessage(
hDlgChat, IDE_TEXT_BUFFER, EM_REPLACESEL, false, (LPARAM)textbuff);
}