What is the etiquette for asking a professor to waive the prerequisite for a course? Should I mail the professor? Should I meet him in person? I am a MS student in CS who wishes to take a graduate level course in Statistics department. The course is Machine Learning and I did cover most of the prerequisite course's content in multiple undergraduate courses so I want him to waive the requirement. I have budget for only 10 courses and wish to complete this degree in 2 years. Taking these prerequisite course would mean that I cannot take other courses that interest me. 

The other reason I wish to have the requirement waived is because the prerequisite course itself is a statistics department course which also has prerequisite statistic courses and my college only allows me to take 2 courses outside of our department.
Go in person. Tell him what you told us. Be prepared for a few quiz questions about your stats knowledge. Answer them correctly. You might not get any questions, but you should be able to describe, at the very least, what your prior classes were about.