Calculate Frequency/Period from Output of a Maximum Entropy Spectrum MATLAB function
From the code on p.$7$ of this document it can be seen that the $N$ (nfreq in the code) frequency points are chosen equidistantly between zero and Nyquist (i.e., $f_s/2$, with $f_s$ being the sampling frequency) using the command linspace(). So the individual frequencies are given by

$$f_k=\frac{kf_s}{2(N-1)},\qquad k=0,1,\ldots, N-1\tag{1}$$

[Note that the indices $k$ in $(1)$ start at zero.]