The directory Macaulay2/book contains the code for producing the documentation in TeX form, which can be printed with dvips or viewed on your screen with xdvi. The hypertext links in the book can be followed if you get xdvi version 20a or later. (If your old xdvi complains about the unknown special commands that implement hypertext links, you can give it the option -hushspecials to silence it.) The source code for xdvi can be obtained at one of the CTAN mirror sites in the directory dviware/xdvik. Obtain a list of CTAN mirror sites by fingering ctan@ftp.tex.ac.uk.
Finally, all the documentation can be viewed within the program in text form using help.