If you didn’t create a PlanetJune account, and used the PayPal Express Checkout, my shop would have created an account, and a temporary password, for you. You should have received an email with the password – if you didn’t receive it, it’s probably in your Spam folder.
Please check your Spam, and if you don’t find the email, click ‘Forgot your password?’ from the login screen, enter your email address (the one you used with your PayPal account) and my system will send you a new temporary password (again, check your Spam if you don’t see the email). You can change your password to something you’ll remember once you’ve logged in!