Be sure you're using the login box at the top of
this page (as opposed to the signup page you get after clicking "Register"). According to the error message, your oekaki account already exists, so try logging in with the password you chose when creating the oekaki account (as opposed to your forum password). If that doesn't work, try using the "recover password" link to reset your oekaki account's password.
If you still are unable to log in after trying the steps above, please let me know and I'll help you get your account working.