How should I approach a Tokyo University professor to join their lab?
For professors in Japan, taking on students is a tremendous responsibility and burden. Unlike in the United States or Europe, professors are responsible for even the extracurricular activities of students (i.e., getting arrested; showing up drunk and groping someone; having an apartment so messy that the landlord complains, etc.). They are also responsible for the student's career after they graduate. 

So faculty are noticeably reticent to take anyone on that they do not know or that do not have anyone to vouch for them. It's in this context that letters of introduction from known faculty are extremely important. 

This is very hard for outsiders to break into. Your best hope is that one of your professors knows somebody at U-Tokyo, or knows somebody who knows somebody at UTokyo. It's their letter of introduction that will open doors for you.

Cold-calling will not yield many good results. 

Without a good letter of introduction, your next best strategy is to enroll in either a study-abroad program / exchange program with U-Tokyo that has open enrollment or to apply to one of their English-speaking graduate programs. Going through their international programs office tends to be much better as the staff are used to international norms for student applications. Unfortunately, most of these programs aren't in engineering, but at least it gets you a  foot in the door.


Edit: From the discussion, it became clear that the OP has already enrolled into an engineering program at U-Tokyo that caters to foreigners. In this case, it's the program's responsibility to find him his advisor, as it is clear from their FAQ: 


  "As an applicant for all courses, it is not necessary for you to
  contact a prospective supervisor in advance. Instead, after evaluating
  your application documents, we will allocate you to a field of
  study/faculty member which will be most appropriate for your research
  and research interests. "