The example works like this: ExamTemplate.tex is the file you run TeX on. It contains things like the name and date of the exam, the exam instructions and the name of the file with the exam questions in it. The file ExamQuestions.tex has the questions themselves in the order in which they are to appear. You shouldn't need to modify DoPlainExam.tex; it has the macros themselves that make it all go. When you run the example, keep all three files together, since ExamTemplate.tex calls the other two files.
Most of the features of the DoPlainExam.tex macros are shown
in the example. Among them are: automatic problem numbering,
creating an optional grade grid at the beginning of the exam
to record scores for each problem and a total, automatically putting the
number of pages in the exam at the top of every page and in
the directions on the first page (making it easier for a
student to know if an exam is missing the last page), and
simplified formatting (you just enter the text of each
problem and the amount of space to be left after the problem
for the student's answer). The macros also allow you in a fairly
easy way to make local changes in font sizes.