When you create an account using your social network account there is no password for your account by default but you can set it yourself.
To do this slide your cursor to the account - settings as on the screenshot below.
In the "Change Password" settings you need to click "Reset"
You will be automatically logged out from your account on the website and redirected to the page where you need to enter e-mail that was displayed on website.
After this you need to log into your e-mail account and locate an e-mail with a link to reset your password.
Click on the link in the e-mail and you will be redirected to the page where you can set a password for your account.
Click "Reset Password" the following window will appear:
Now you can login under the e-mail + password.