McMaster's website does tend to be hard to navigate but here's what I found:
According to the Welcome Day website you just meet at the lobby of the building you want to tour during 9am-3pm, there will likely be people waiting there / signs:
http://fye.mcmaster.ca/wdevents.html
According this page on the registrar's site:
http://registrar.mcmaster.ca /tours.html there are residence tours on Monday August 9 and Friday August 13th. You have to use the web form available at:
http://registrar.mcmaster.ca /phpforms/newtourform.php
to prebook. Make sure to select residence only tour if that's what you want. It will also let you select the date once you select the desired type of tour.
Also note that the full campus tour includes stops in some of the residences as well.
Good luck and welcome to McMaster
.