I've tried to do this a couple of ways but it dosen't seem to work or atleast the quote doesn't look different when I preview it. I'm sure it's something quite simple but darned if I can figure it out.
1 - hit the "quote" button in the top right corner of the post you want to quote.
2 - copy the text you want and paste it into a new post. then highlight it and hit the "quote" button above (by the bold, ital, underline, etc. buttons)
3 - manually put in the quote command by doing an open square bracket "[" followed by quote and a close bracket "]" at the start of the text and the same but with /quote at the end.