At my first University I worked on campus, it's pretty flexible because the good part of working on campus is that they understand school comes first. At my second university (I know, I've been to a lot of schools lol) I also worked on campus, both times I worked with the events squad however at my second school they wanted you to prioritize work over school. So it just all depends on who you end up working for.
I plan on working at Mac as well, you have to log onto Oscar Plus and click on workstudy to see on campus job opportunities and apply for jobs. However none of this will be available until August 1st.
Here's more on how to apply:
http://sfas.mcmaster.ca/work_study/jobs.html