Does HOMER take into account the outdoor temperature to correct the air density in the wind power calculation or is the "actual density" refer to the height above see level density adjustment only?

There are two ways that you can adjust for colder temperatures.  One would be to figure out what the equivalent altitude would be for the same density.  A more sure-proof approach would be to create a new turbine power curve.  There is a "New" button in the Wind Turbine Inputs window.  I don't think you should adjust the windspeed because power curves are so non-linear.