Amplitude of frequency in MATLAB FFT
The formula you are looking for is


  (desired freq / Sampling Frequency) * Length of samples = sample number


you can see this works out even by verifying the units


  (Hz / Hz )* sample = sample


depending on your exact frequencies, the result may or may not be an integer number. If it is not an integer, that means the exact frequency wasn't captured (due to Fs). For example if your result was 23.8 you would have to look at either sample 23 or sample 24 to get the data you required, or do some sort of interpolation.

Good luck

edit 

as your comment specified, this doesn't work exactly, the reason is becasue matlab does 1 indexing, not 0 indexing. So our DC component 0Hz is at element 1, not element 0 but the above formulas are correct. However implementing in matlab is a little different

desired_freq = 120; %Hz
desired_index = (desired_freq / Fs) * L + 1; %+1 is because matlab uses 1 indexing not 0 indexing


again desired index may not be an integer number so you can't simply say mag(desired_index) one workaround would be mag(round(desired_index))