Yes you do. If you fill up your profile in the Graduation Information Centre. You pay as follows:
$40: if you are attending your convocation and getting your diploma on the day.
$25: if you're not attending your convocation but want your diploma send by mail
Free: if you're not attending your convocation but are able to pick up your diploma in the Registrar Office
I hope you already signed up because the deadline was May 30. More information, look here:
http://registrar.mcmaster.ca /category/convocation/
Oh yeah, make sure you don't have any outstanding fees before graduation which is your student account.