McKinsey Interview and Other Questions

McKinsey's first round interviews claim that I have a 1-hour multiple choice problem solving test. I was wondering if any of you have experiences with this and, if so, how difficult it was. What sort of materials were covered?

Also, when I applied to McKinsey, I listed an international office as my first choice. However, I now wish to work in New York. Would it be possible to change my preference and would this damage my impression to the firm?