After purchasing a course, an email with the login info should have been sent to you. If you haven’t gotten one, try checking your spam folder to see if the login email ended up there. It’s rare, but it happens.
If you still can’t find it, the quickest solution would be to reset your password by entering your username (the email you used when purchasing) here: http://bloopanimation.com/lostpassword/
If the password reset function isn’t working properly, try emptying the browser cache, or switch browsers. That usually solves it.