[pxx, w] = periodogram(xx);
gives you the PSD (pxx) and the normalized frequency (w).
If you are falimiar with the theory, you can convert it into the frequency of your expectation as follows:
is the formula. N is the number of samples.
If not, the best bet is
[pxx, w] = periodogram(xx, fs);
where fs is the sampling frequency and w is the converted frequency.
Also, you should look into how PSD is calculated and it would help you understand how the PSD data are sorted.
You cannot expect something you find in your text book to appear.
If you do not have time to learn it, then you can just copy the way it is used:
