Okay, sounds like you're on the right track. Now, most DLC isn't free, so you've got to pay to play. (Patches are always free)
"I can't get access to them" is a little to vague. What error or message are you getting? Is this DLC that you've already DLd or something that you' re trying to get now?
I knew things weren't free. I am fairly new to xbox live, but I've dealt with it a little in the past. Based on some searching, I think I am figuring out the problem. Apparently other people have these problems. When I go to the games marketplace, I select add-ons. Sometimes, it gives me the message "content not available." Then sometimes, I am able to get a list of add-ons. Then I select one, say Dead Money, and a message and status code come up. The message says "Can't retrieve information from xbox live. Please try again later," and the status code is 807B01F4.