Where in the preamble to the Declaration of Independence does it talk about separation of powers or government with branches?

Separation of powers is not a concept that is discussed in the Preamble to the Declaration, (or elsewhere in the document.) Generally, the Declaration is an assertion of principles and a list of grievances against King George. Their focus is on the alleged abuses they have suffered as British subjects, not how government might be structured in such a way as to avoid these abuses. Their complaints were related to the imperial relationship between the colonies and the Crown and Parliament. They argued that the King (after spending years arguing against the usurpations of Parliament) had violated their rights as Englishmen. The idea of separation of powers was well-established in the Anglo-American world--the famous French philosopher Montesquieu had based his defense of the concept on the English constitution--but Jefferson and the authors of the Declaration did not assert it as a principle in the Preamble or elsewhere. To see what late eighteenth-century Americans thought about the importance of separation of powers, one might turn to the various state constitutions, which established the principle after the Declaration, or the Federalist Papers, specifically Federalist 51, which discusses and defends the system separation of powers in the Constitution.