Certainly you should go if you have an interest. Not only will it give you a better sense of what Hopkins is like but also a better sense of what an American university is like.
What can you expect? Probably a brief slide show or video with views of the campus, an explanation of what is required for application with perhaps some statistics and hints about what makes a strong application, probably some time on topics of particular interest or confusion to UK students, and an opportunity to pose questions. There might be time to meet the presenter afterward, too.
No, you don't have to have questions, but you could a few prepared in case they are not covered. And it doesn't hurt to introduce yourself to the presenter after the session is over, either, maybe with a question of particularly personal interest. Oftentimes s/he will be the one reading your application should you decide to apply.
Some suggested questions:
--What kinds of adjustments do you see UK admits needing to make?
--What are residence halls like (single or shared room, mixed gender, mixed age/class)? How many students live on campus? Where do those who don't live on campus reside?
--What is your four-year graduation rate?
--What are the prospects for an international student to receive financial aid?
--Do students tend to stay near campus or do they often leave for class/work/entertainment? Is public transportation easily available?
--What is it like to study <your area of interest>?