A question about the PSD unit by using FFT method.

I'm just doing a power spectral density analysis of a signal in time domain. I'm following the fft method described in :

http://www.mathworks.com/support/tech-notes/1700/1702.html

It gives the real physical unit for the PSD. However, the unit is "power", is that mean "V^2/Hz"?

If I take 10*log10(power) or 10*log10(V^2/Hz), do I get the unit of "dB/Hz"?

Then how can I convert it to dBm/MHz?

Thank you very much!