The spectrum of an amplitude modulated signal as you describe will be made of two peaks located at frequencies :
(carrier freq + message freq) and (carrier freq - message freq)
So, we should observe a peak at 29 kHz and a peak at 31 kHz when using the formula and values you mention.(Pls see attached VI)
If you are using a sine wave signal as a modulating signal, you may try to use the "Extract single tone.vi" in order to get the frequency of this signal - That's what is described in the attached VI.
You also need to make sure you are using appropriate sampling frequencies (>60kS/s) in order to respect the Nyquist criterion.
I hope this will help.
Gerald