A login name is a sequence of alphanumeric characters and underscores.
You cannot alter your LoginName: you have to get the administrator to do this for you.
Look in WikiUsers - if your name is followed by a dash and a word, usually lowercase, this is what you log in with.
Would mean that WikiGuest would log in using the login name 'guest'
If, on the other hand, if your entry is like this:
Then you would log in using your WikiName, in this case 'WikiGuest'