Finding the dominant tone in a signal
I think you are having trouble constructing your frequency axis properly. Once you do this, you can do a simple peak pick. I have re-written the code for you:

Fs = 48000;
x = dataSet;            % 256 samples data set
fftLength = length(x);  % Always make sure to be at least as long as your data. 
xdft = fft(x,fftLength);
maxAmp = max(abs(xdft));

freq = [0:fftLength-1].*(Fs/fftLength); % This is your total freq-axis
freqsYouCareAbout = freq(freq &lt; Fs/2);  % You only care about either the pos or neg 
                                        % frequencies, since they are redundant for
                                        % a real signal.

xdftYouCareAbout = abs(xdft(1:round(fftLength/2))); % Take the absolute magnitude.

[maxVal, index] = max(xdftYouCareAbout); % maxVal is your (un-normalized) maximum amplitude

maxFreq = freqsYouCareAbout(index); % This is the frequency of your dominant signal.