Not all fonts have the letter M in them (for example Chinese), but all fonts have a height. The term has therefore come to mean the height of the font - not the width of the letter M. In CSS, the em unit is a general unit for measuring lenghts, for example page margins and padding around elements.
Boston University's largest study abroad program is located in London, England. Boston University London Programmes offers a semester of study and work in London through their London Internship Program (LIP), as well as a number of other specialized programs.
The University Student Union is located near the center of the CSULB campus, as you can see via this map (the building is marked USU). As Dexter walks towards the entrance of Brunner Hall of Science (a fake sign, of course), he is walking south, towards the north entrance to the building.