Get verified (tiers 1 and 2 are very simple and quick to do), then do a wire or sepa transfer using the deposit page. Then you can trade with the fiat you've deposited, and then do a withdrawal.
This is basically right, but the only fiat we offer for trading right now is EUR. The only way to deposit USD is to have your bank or our bank convert to EUR for the deposit. For the withdrawal, it'd have to be your bank that does the conversion back to USD since our bank won't do that.
This is a temporary situation since we'll have USD deposits fairly soon (say within a month if all goes well - and yeah, it might not).