How does a pilot report a person (on the surface) in distress?
I would treat an active SOS signal from the ground the same way I would treat an ELT signal on 121.5: Contact ATC (or Flight Service if you're not sure what ATC frequency to use) and inform them of the event.

As with reporting an ELT you should be prepared to tell ATC:


What kind of signal you've observed
(ELT, signal light, signal mirror, "SOS" or "HELP" spelled out with rocks…)
The time you observed the signal
(Ideally you'll report it while overhead &amp; actively monitoring it)
Your location and altitude


Ideally you'll be able to give your location as a GPS fix as SHAF described.
If that's not possible there are other options, in roughly-descending order of accuracy:


A DME fix (radial &amp; DME distance)
A VOR fix (the intersection of two radials from separate stations)
A VOR radial &amp; visual landmark (ideally something on the sectional chart)
While just a radial is not very useful it can be helpful in conjunction with a landmark: "South side of Mt. Greylock, on the 350 radial off Chester VOR" helps narrow down the search area considerably.
A visual landmark based on the sectional chart.


ATC may also ask you to circle the distress signal and ident in order to get a radar fix to supplement any of the above.