JSOP, AppendText actually goes all the way back to .Net 1.0. It is just not available in .Net CF, which this person is likely using. Like I said before, the solution is to do:
txtDataReceived.Text = txtDataReceived.Text + txtDataToSend.Text;
That will not cause a compile time error. If it does, then post what that error is and we can assist you further. You have only posted the error for AppendText, not for this line of code.