Yes, Feralheart does indeed work for Windows 8 by what I've heard.
If you have numbers in your username, make sure that you're using the numbers above the letters, not the numpad, and also make sure that you type everything in very slowly and that you don't have caps lock on. If your username is longer than 16 characters, the server won't let you log in and you'll sadly have to create a new account.
Make sure that you have a good internet connection as well. Hope this helps!