The Homestead Act is a decision available to 
USA.
It will be listed after 1862 if USA is a democracy, and it can be taken if USA has at least 40 prestige.
Normally USA should easily have 40 prestige by 1862 due to prestige from the Mexican-American wars, colonizing the west and the civil war.
The decision will give USA a +50% modifier to assimilation and immigrant attraction for the rest of the game. USA is already by far the nation, that receives the most immigrants due being in the new world, and this will only solidify the position as the number 1 destination.
All in all it is one of the best and most powerful decisions in the game.