Jack,
Since your question is short on specifics, I need to make some assumptions. If you are using the basic FFT function, look at the help file for a definition of the frequency bins. Some of the higher level spectral analysis VIs have a df output which makes determination of the frequency bins very straightforward.
Once you determine the frequency bin width, you just count up to the desired frequency and look at that element of the FFT array. If you have bins that are 2 Hz wide, then 50 Hz will be in the 25th bin (index = 24). Depending on what resolution you have and what kind of windowing was done, the adjacent bins may also have some of the energy of the desired component.
Lynn
