First of all, let's be clear that we're talking about logging in to the Cyc Server, not the machine on which the Cyc Server is running. The purpose of logging into an Cyc Server is to identify the instance of #$Cyclist which represents the author of operations generated by the interface. Whenever a WWW connection is made to an Cyc Server, all operations must be associated with some #$Cyclist.
Go to the top frame of the interface and click on "Login", and the Login Area page will be displayed. The page will tell you who you are currently logged in as, and any others who are logged into the same Cyc Server.
Unless someone has already logged in to this Cyc Server from your web browser, the cyclist should be #$Guest. #$Guest is an instance of #$Cyclist which is treated specially by the system: while logged in as #$Guest, you may not perform editing operations or other actions which could modify the KB.
At a given site, the people who edit the KB are represented in the KB as instances of #$Cyclist (and, one would hope, as instances of #$Personand other relevant collections).
If there is an Cyc constant which represents you, you can log into a OpenCyc® Server from the Login Area page by entering the name of your constant and clicking the "Submit" button. The KB editing facilities are then enabled and any Cyc operations which you originate will be tagged as yours.
For example, if Mary Jones is represented by the constant #$MaryJones, she can log in by typing "MaryJones" (no "#$") in the input field and clicking "Login".
If you are initially logged in as #$Guest, you'll need to find someone who already has their own Cyc constant who can login first and assist you, since #$Guest does not have KB editing privileges. New Cyc images (KBs) come with only one login name other than #$Guest, and that is #$CycAdministrator. Log in to this account in order to then create other #$Cyclist accounts.
The changes that you make to the KB on your Cyc Server are not automatically shared with other Cyc Servers, or recorded in the "official" KB maintained at your site. For example, if someone who already had a Cyc constant created your constant for you, and you wanted it to become part of the site KB, you would have to explicitly instruct the Cyc Server to share its operations.