Think of Major as a patriot in the days of the American Revolution. The American revolutionaries were upset about taxation without representation, being forced to give over money without having a say in how it is used. Major is similarly concerned. The animals are being forced to give over their time and effort, with no say as how it is used, when it is used AND no opportunity to really benefit from it.
On your subquestion - with a question like this, the best way is to not focus on detail. Identify the subject of the question (Major) and identify the information requested (complaint). Consider the book, and identify the character. Then, list everything you know about the character. Then look at your list and see if any of it fits the information that is being requested.
Major's main complaint about the farm is the way that the animals' lives there are structured. He thinks their lives are markedly unfair, and that "Man" (humanity) steals their labor and their lives.
He feels that the humans are getting rich off of the animals labor and lives. The animals work the hardest while getting no reward. The humans hardly work but get to reap the rewards.