By simply clicking on the Reply "button" in the lower right of the person's post you want to quote the box will appear with their quote in it. Can I suggest if the post is large that you delete the parts you don't need to make reading threads easier. Anything between "QUOTE" will appear.
Another option is to highlight part of the post, from left to right, right to left doesn't work for me, and a little "quote" box will pop up. Click it and it will start a reply box for you.