Trac can be configured to authenticate using LDAP, Apache or a user list for authentication for users.
If Trac is configured to use an internal user list, i.e. it is running under it's own internal webserver (called 'tracd') then a simple text file controls user access and is normally found under the trac project directory. See http://trac.edgewall.org/wiki/TracStandalone#UsingAuthentication
The above link will be useful to look at regardless of the authentication method.
If when you log in to Trac, you use your normal Windows login credentials then it is configured for LDAP authentication and you don't usually need to add individual users manually.
Also when initially configuring Trac, you do need to set up an admin user which you have to do via the command line (it sounds like this has already been done):
permission add <username> TRAC_ADMIN