As a collection of narratives, historical descriptions, and interviews, To Be a Slave does not fit into a typical plot diagram. Each story has its own hooks, as well as rises and falls in the action, and its own direction and purpose.
If you look at the book as a story of slavery in America as a whole, however, you can see something of a "plot diagram" if that's what you need to do. You might look at the stories of life in Africa as the exposition, the initial capture into slavery as the narrative hook, and the Civil War as the climax. That means that everything from that time of being taken into captivity until the war can be seen as rising action. That includes stories of slave auctions, adjustments to life on plantations, master-slave relationships, work and responsibilities of slaves, escape attempts and successes, punishments, religious experiences, etc.
Taken this way, the falling action can viewed as the time period immediately following the final battles of the war and the South's surrender, and the Resolution as the granting of emancipation and the rights of citizenship to African Americans (though that resolution continued to be worked out for many years).