I have a mailbox account and a mailbox pin but I cannot access the mailbox
Which mailbox are you referring to?
Do you mean the account messages from Fizz? These go to the inbox of your own email.
Do you mean the community messages?
If you are referring to the voicemail messages of Fizz mobile, you can see the instructions here: https://fizz.ca/en/faq/how-can-I-access-and-use-my-voicemail