Another for the FAQ!
Someone else could probably explain this better than I can, but what I do is go to the Library home page and find the 'search' field near the top. I hit 'advanced search' and it sends me to another page where there are many options for how to search (date, size, story codes, author, etc.). Type in what you guess to be the author's name, scroll to the bottom of the page, then hit 'return' (or 'go', 'search' or whatever the button down there says).
An alternative would be if you're certain about the title of one story the author has posted, in which case instead of hitting 'advanced search', type in the title and you will be taken to a saummary of the story, which includes the author's name. Hit the name and you'll be taken to a list of their stories.
Note Bene: Many authors use different names in the Forum and in the Library (Phoenix and Firebird, Couture and S_Couture, boccaccio and Big Jake, etc.). I just had to use the second method to track down nova, who writes under an entirely different name.
Good luck!