Evernote on Mac

Hi All. I created an Evernote account online and signed in to the apps on iPhone and iPad no problem. However I am having trouble signing in to the Mac app. On opening the Mac app I am presented with a window with text fields for email address, username and password, but these only seem to be for creating a new account. There is a Register button, but no button or option that I can find for signing in to an existing account. There are also a lot of unlabelled buttons. If I enter my account details and press the Register button to see if that'll work, it doesn't seem to go anywhere. Can anyone help? Many thanks.