If your PosterMyWall account is connected with your Facebook, you will not be able to log in using your email. You can change this by setting up a password for your email you used to register.
To set a password, go to 'Edit Profile' then click 'Set password to login with email address'. Set a password and confirm it in the provided spaces. Save the settings, and you're all set. You can now login using Facebook or your email.
To completely disconnect Facebook from your account. Go to 'Manage Login Accounts' in 'Edit Profile' and disconnect Facebook.
You can always reconnect your account with your Facebook or gmail account form this option.