OAuth requires that each request be signed using either RSA-SHA1, HMAC-SHA1 or plaintext (see Section 9 of the OAuth spec). Google supports either HMAC-SHA1 or RSA-SHA1.
In order to use these signature methods, you must first register your domain with Google. You can register or view your values for the consumer key and other signing variables here: https://www.google.com/accounts/ManageDomains If you don't want to register a domain, select the "default signature method" option to use the sample credentials.
Using the RSA-SHA1 signature method requires the following fields:
Consumer Key: This is the domain name you have previously registered with Google.
Private Key: You must generate a public/private key pair and upload the public key to Google. Instructions on how to do so can be found here. In order to use the private key with the Google Data Java Client, the private key must first be converted to a Base-64 encoded PKCS#8 formatted string. You can do that with the following command:
openssl pkcs8 -in myrsakey.pem -topk8 -nocrypt -out myrsakey.pk8
The string in "myrsakey.pk8" should be used in the Private Key field below.