Berkeley professors hold regularly schedule office hours during term time, so that students can come by without an appointment. Office hours should be listed outside professors' office doors. The mechanical engineering staff also posts a list of all professors' office hours on the department website, but usually not until around the third week of classes. Professor McMains also posts her office hours on her website under the Instruction tab.
You do not need an appointment to come to office hours. However, to be safe, you might send an email if you are not taking the professor's class stating that you are planning to drop by office hours on [day] to discuss [x] and to please let you know if there is any problem, otherwise you will see them on [day]. That way the professor can warn you if that day is right before a midterm or such, when office hours are likely to be very full, or if office hours were rescheduled that week.
If you have conflicts with all office hours, then a good way to schedule a meeting is to provide a list of all your openings for a given week (or a screenshot of your electronic calendar or schedule) and ask the professor to suggest one of your open times that is convenient for them for a meeting. Include your phone number, because arranging meetings with a long back and forth by email can be very inefficient.
(If you have taken a class with me, you will recall from the syllabus that I have an RSI from too much typing and asked all students to always include a phone number in all emails.)