You need to select your range first:
int result = 0;
FINDTEXT findtext_param;
RichEdit1->Text = "Hello,this is a test from younth.";
char find_str[256] = "from";
findtext_param.lpstrText = find_str;
findtext_param.chrg.cpMin = 0;
findtext_param.chrg.cpMax = -1;
CHARRANGE cr;
cr.cpMin = -1;
cr.cpMax = -1;
SendMessage(RichEdit1->Handle, EM_EXSETSEL, 0, (LPARAM)&cr);
result = SendMessage(RichEdit1->Handle,EM_FINDTEXT,FR_DOWN,(LPARAM)&findtext_param);
if(result >= 0) ShowMessage("string found.");
else ShowMessage("string not found.");