In what way is freedom expressed in a Humanist society?

I do not think that any Western society has ever been completely Humanist.
Humanism, as you likely know, is a school of thought that emerged during the Renaissance in the fifteenth century. It rejected the absolute authority of the Catholic Church, discouraged superstition, and instead, supported the notion that most of civilization's problems could be solved through rationalism. Additionally, Humanists did not see human beings as innately sinful. They elevated humanity -- the beauty of the human mind and human forms, which were depicted and celebrated in the arts and letters of that period.
Renaissance painters and sculptors -- most notably, Michelangelo and Leonardo Da Vinci -- espoused Humanist ideas. They surpassed the flat simplicity of painting from the Middle Ages by studying human anatomy and physiology. Their interest in the body allowed for the creation of more realistic forms and movements. Da Vinci went further, using sfumato in his paintings, a technique that experimented with the illusion of light by allowing tones and colors to blend into one another, which also produced softer forms and the illusion of distance.
Da Vinci's interest in the sciences also led to plans for inventions, including an early version of the helicopter and a more efficient clock. 
Humanists expressed their freedom by being interested in the world around them and exploring its potential -- in every discipline of learning. The Catholic Church disapproved of much of this. While it had sanctioned art works -- as those still depicted Biblical figures, though in Classical forms -- they resisted experiments with science, especially ideas that challenged the geocentric belief.
Humanists were secular. Secularism has, and continues to be, at odds with religion.