Ethfinex is a decentralized ETH exchange made by the very Bitfinex. You will not be able to use it unless you have Bitfinex account and as ive said in last post - this exchange closed signups along with few others - https://steemit.com/bitcoin/@kingscrown/register-to-more-exchanges-more-and-more-are-closing-doors-for-new-user
Now here is how the exchange looks like - pretty much same as BitFinex but only with ETH pairs.
Despite launch today i didnt find anyone sharing insides of it so we can assume we are first!