What you want is a hygrometer, not a hydrometer. Hydrometers are meant for measuring the specific gravity of a solution.

The hygrometer you bought is working. What they don't tell you is to get the humidity, the unit itself has to be inside the incubator. The probe only measures temperature.

Keep the humidity around 50 during incubation, and up it to 70 to 80 the last 3 days for hatching.

I also didn't know the whole thing had to be inside the incubator until my son pointed it out to me after a couple of weeks. I thought it wasn't working.

Also, get your self one of those pool toys at Walmart that looks like a plastic cylinder about 6 inches long, filled with a gel, that you can put your finger inside. Some people call them Water Weasels. They sell for under a dollar.
Put the probe inside it in the middle, and put it inside the incubator. That gives you the most accurate temp of the inside of the eggs.

M