Unlike at some public universities, UW does not generally admit freshman directly into specific Colleges within the University. Most students start in the College of Letters & Science and, at their freshman orientation (called SOAR) – which international students do just before the rest of the freshman move in – advisors will work with you to develop a schedule which starts you on the path of meeting the prerequisites for admission to your desired program, whether that is College of Engineering, Business etc.
See what you can do to maximize your IB scores so that you can apply for credit for those scores both to meet general education requirements and to pass out of intro classes if that would make sense for you, given your academic goals.
As razed1 noted, no, you do not need to physically travel to WI to take the placement tests. They are given online and the test scores are used to determine whether a student places out of basic competency in english composition and math skills, as well as placement for math courses. Each test is an hour. There is also a foreign language placement test for students planning on continuing with their foreign language. Well worth taking that as well, though I cannot recall if IB scores can be used for foreign language placement. I would imagine that information is on the website.
Coming in with credits from IB is useful as priority in class registation for second semester and beyond is based on how many credits a student has completed. So, if a second semester freshman has a ton of credits from AP/IB testing, they get to register before students with fewer credits, preserving the ability to craft a better class schedule.
Congrats, and enjoy.