There is no registration. There are no accounts, and there are no passwords.
Your username is what you want your worker name to be with and what your payout address is put together by '@' character. So if you want your worker name to be "coolermaster" and your payout address is "D5EPJTJ1vRXU7h1qVjpJk4QzByp4n581c7", then your username would be coolermaster@D5EPJTJ1vRXU7h1qVjpJk4QzByp4n581c7
Your worker name cannot contain any special characters. Only alphanumeric characters are allowed. Worker name's length must be at least 1, and at most 25 characters.
Make sure to copy and paste your wallet address instead of typing it. If you are being denied login, it is probably because your address is probably wrong, or because you have a special character in your worker name
Password can be anything. Put in whatever you like.
If you haven't mined before, go to https://litecoin.info/Mining_tutorials and read up on how to start mining.