I think you're good.
The last day to add/drop term 1 courses without financial penalty is May 6, 2011
The last day to cancel term 1 courses without academic penalty is June 1, 2011
The last day to add/drop term 2 courses without financial penalty is June 24, 2011
The last day to cancel term 2 courses without academic penalty is July 20, 2011
The last day to add/drop term 3 courses without financial penalty is May 6, 2011
The last day to cancel term 3 courses without academic penalty is July 20, 2011
Source:
http://registrar.mcmaster.ca /registered/