You aren't allowed to post comments, but you are allowed to post answers.
The way to do that at the very bottom of the page:
(You could say it's somewhat hidden, since the "add a comment" link is much closer to the question and the top of the page, especially if there already are some answers.)
If you have an answer, write it there. (And be sure to check the editing help, too).
Yeah, that box looks a lot more serious than the small "add a comment" link, but you probably shouldn't let that matter too much. Even answers without all the details are answers. They just might not get that many votes, but answers that are at least correct tend to get at least some upvotes.
Comments are not the same as answers, they're mostly meant for asking clarification, constructive criticism or stuff like that. They're not questions or answers by themselves, but are meant as a tool to make the questions and answers better. Also, you don't get any reputation from them, and sadly they cannot be voted down on.
You can't see it without having enough rep to comment, but the box for writing comments also says: "Use comments to ask for more information or suggest improvements. Avoid answering questions in comments."