MtGox API Overview and explanation of steps required to create connection
Take a look at the Java XChange project

While it is not Objective C it will provide you with all the details you'll need to get your library off the ground including the HMAC authentication process and websockets. 

You'll also find a range of other exchanges supported so you can implement those as well thus providing your application with a broader range of potential users since not everyone uses MtGox these days.