Green Party Won't Be Heard During the Debate - Unless You Make Yourself Heard Now
Here is the Green Party petition on the matter, and here is a petition you can sign to express your displeasure.
And thanks to Chris, here are some people you can call and express your support for the Green Party's right to be heard:
Or web/email: http://www.cbc.ca/contact/index.jsp
Radio-Canada Nouvelles (news):
Phone: (416) 332-5000 (Ontario)
Phone: (204) 956-2025 (Manitoba)
Global National TV
Finally, the debate is accepting questions from 'the floor' so you could email the moderator at: firstname.lastname@example.org suggesting that they ask the parties what it was about the Green Party that scared them so much they had to exclude them from the debate (or make up your own question on the topic - perhaps something less confrontational is more likely to be aired (e.g. Do you feel that the Green Party should have been included in this forum, why or why not)).
Oh, you're wondering why the Green Party should be included in the debate? I saved the actual argument for my next post, so those who are already on board could get on with it and start harassing people.
Note: post updated to include Green Party petition.