how get privateKey , if signer name is known ??

i have wrote:
dsa = Signature.getInstance("SHA/DSA");
signer = new SystemSigner("Administrator");
priv = signer.getPrivateKey();
dsa.initSign(priv);

It is known, i have a "Administrator" certificate
An "java.security.InvalidKeyException" happened.(DSA private key is null)

Please , tell me if you know , or mail me: [email protected]
Thanks

have u generated ur keypair? if not, how to get privtekey since u don't have key pair yet. hope this help..., do inform me if this help..