How does Ripple's exchange work? In Advanced/Trade tab there is a "BTC/XRP" box, and a "change issuer" link.
I have no clue what should I do to post any buy or sell:


I can freely modify the exchange currencies, and only USD/XRP shows me an order form
it has a ripple address near "base currency" / "change issuer", so I guess this means that that address somehow takes care of the USD exchanges?


I'm still clueless about what am I supposed to do to either set up a Bitcoin exchange myself, or to fine an existing one, neither have any idea how this all works.
I am clueless as well but I think that it requires a consensus of people on the network for this to function properly through one accepted BTC address. When we create our own orders no one else can see it unless you are in their trust circle or ripple neighbourhood. So we would have to all use your posted address to trade in BTC. Maybe MtGox sets up a ripple wallet inside the ripple network then we would all flock to their address and use it as a trusted source for the exchange. I could be totally wrong but that is how I poorly understand it. Someone with more technical know how can explain it better I am sure.

Ripple FAQ - Limited Description of Distribute Exchange