There are two SE wikis: http://se.cs.toronto.edu, which is public-facing; and https://seinternal.cs.toronto.edu, which is for internal use only.
To read the internal wiki, you need to type your CSLab user name and password into your web browser. Anyone can read the public-facing wiki with no password. To edit either wiki you need to log in to the wiki.
Accounts on the SE wikis are restricted to members of the SE group. This determination is made automatically based on who is the sponsor of your CSLab unix account.
To apply for a CSLab unix account, fill out the form at https://csweb.cs.toronto.edu/accounts/request/ and then bug your supervisor to approve the request. Note your "login name".
After your CSLab unix account is created, you will automatically get mediawiki accounts for http://se.cs.toronto.edu and https://seinternal.cs.toronto.edu. To set a password for mediawiki, go to the login page, type your CSLab account name, and press "E-mail password". It will mail a new password to firstname.lastname@example.org. (Accounts are automatically created on this server several times a day, but your POC can accelerate the process upon request.)
If you already have a CSLab unix account, and your sponsor is in SE, you already have mediawiki accounts (but you may have to set your passwords as described above). If you think you should have a mediawiki account but don't, please contact your POC.