Your sampling interval is 1 month, so your DFT bins will be in cycles/month. So you can just set Fs = 1, but then keep in mind that the frequencies will be in cycles per month, so your highest frequency of 1/2 is 1 cycle/2 months.
Also, the code I had given you earlier was assuming you have an even number of samples. If your signal length is odd, you have to make a small adjustment.
Fs = 1;
xdft = fft(x);
xdft = xdft(1:length(x)/2+1);
freq = 0:Fs/length(x):Fs/2;
[maxval,idx] = max(abs(xdft));
freq(idx)
