Go to http://webassign.net/login.html
Scroll down and click on the box saying, "I have a class key."
Type the code for your section listed in http://www.math.mcmaster.ca/~haskell/math1a_10-11/courseoutline.pdf
around the bottom of the first page. Then, choose to log-in if you have a preexisting account on WebAssign or register a new account. Either way, you have to use your MacID as your username, and "mcmaster" as the institution. (You don't have to worry about the password!
Now, you should be enrolled in the online section of the course.
After you log-in, choose your prefix (it should be EWB-MTB) and type the rest of your access code, and you have access to the assignments.