I'm not entirely sure how to do it, as I haven't had much experience in fooling about with iB anyway. If someone could just point it out it'd save a lot of embarassing apologies... :b
In the General Discussion section I have a thread pinned at the top called "The Quote Button" ... that explains it all.
But basically it boils down to hitting the grey button marked "Quote" that's in the top right hand corner of the post from the person you want to quote. There's also an "edit" button, so you can edit your own posts later on, or, if you're a moderator or an admin you can edit other people's posts.