Your instructor will send you an invitation email to their PBWorks wiki. You must either create an account, using your Mason email address as the username and a password of your choice, or log into an account you already have created in the past.
The PBworks course wiki is separate from Blackboard and Masonlive so your Mason password won't work there. You should have received an email from the course wiki directing you to create an account if you're new to it, or to login if you already have an account.
Most professors will use your Mason email address to give you access to their PBworks course wiki. For those who already have a PBworks wiki account using their GMU email address, then you should just have to login with your existing PBworks password (not necessarily the same as your Mason NET ID). For those who did not have PBworks accounts, your Mason email address is your username to create an account with PBworks wikis, so you just have to follow the prompts from the email you get carefully to set up your new account.