The "Last Name" is the last name associated with your online profile. This is not necessarily the last name associated with your account, but rather the last name provide when the profile was created.
The "Email Address or Username" is the email address or username associated with your online profile. If you created your profile recently, this will likely be your email address. If you have multiple profiles with the same email address, you will need to know your username in order to use the forgot password feature.