The wind power class refers to an average wind power density range at a particular height above ground.

For example, power class 4 means that at 50 m above ground, the average wind power density is between 400 and 500 W/m2.  The average wind power density is a function of the wind speed distribution and the air density.  Where the table shows the average wind speeds corresponding to the different wind power classes, they are making some assumptions about both the wind speed distribution and the air density.  Namely, they are assuming a Raleigh distribution, which is a Weibull distribution with k=2.  For times when you don't know the actual wind speed distribution, k=2 is a fine assumption and everybody uses it.  They are also assuming standard air density (sea level, standard temperature and pressure conditions).

With the HOMER DLL, you can specify the wind resource using the annual average wind power density.  The wind resource object has a function called SetAnnualAverageWindPowerDensity, into which you pass the wind power density in W/m2, the altitude in m above sea level, and the Weibull k value, for which you can use 2 as I said earlier.  When you call that function, HOMER assumes every month is the same, and it calculates the air density using the altitude and the US Standard Atmosphere.