Hi Joji,
Since I'm not involved with payment for Madskillz, I'm afraid I can't answer your question about whether you need to bring proof of payment. I suggest you post your question on the Facebook Madskillz webpage. Common sense tells me, though, that you should bring whatever proof of payment you have.
About bringing juggling equipment, I suggest that you do. Of course, everyone there will have equipment with them, but if you don't know anyone personally, it might be weird to ask to borrow something. Besides, you'll want your own balls to take home and use while practicing all the cool things you learn! Clowning Around has a set of three juggling balls for only $6. They may not be the highest quality balls, but they are definitely good enough to start with. Another option, maybe the best one, is that you buy balls or whatever else you might want from a vendor at Madskillz. There are certain to be some good vendors there.
Are you free this Sunday from about 2:00? Would you like to get together with some jugglers? If so, keep an eye on the VanJug webpage because on Saturday I'm going to announce where a group of us will be meeting. We meet different places depending on the weather.
I hope to meet you soon!
Tara