I need to write an argument essay on how the ideas Thoreau presents in "Where I lived, and what I lived for" can, or cannot help people understand and manage their lives. I'm not exactly sure what the question is.

The writing prompt isn't actually asking you a question. It is telling you to write a response that explains why you think that Thoreau's piece is helpful to modern-day readers or not helpful anymore. You are welcome to write a response that goes either way; however, I would recommend writing an essay of support. More than likely, your reader/teacher is a fan of Thoreau, and you might not want to antagonize a reader who will be giving you a grade afterward.
For your essay, I would use the following quote as the foundation of your response.

I went to the woods because I wished to live deliberately, to front only the essential facts of life, and see if I could not learn what it had to teach, and not, when I came to die, discover that I had not lived.

This quote can work for your argument in a lot of ways. Thoreau went into the woods because he wanted to simplify his life. He wanted to pare things down to only the essentials and see if he could learn from that. There's a lot to be said for that. It is very anti-materialism, a tenet that is the basis for modern-day movements like the tiny house trend and minimalism. Thoreau is arguing that living and doing is more valuable than living and owning. Thoreau's quote also shows that living out in nature is a key component. I believe that you could argue that people currently believe this, with or without having read Thoreau, and that is why RV sales continue to climb and why National Park visitor numbers have increased in recent years.