Use your public key as username and private key as password from your Magento connect account

https://www.magentocommerce.com/magento-connect/customer/account/

Connect -> Developers -> Secure Keys -> Create A New Access Key(Or using exsting key)

Also, you can find the public and private key at:

https://developer.magento.com/customer/account/

Developer Portal -> My Access Keys -> Create A New Access Key(Or using exsting key)