You have to create keys of your own. Simply follow the tutorial from the projects home page:
mkdir -p /tmp/keys
keyczart create --location=/tmp/keys --purpose=crypt
keyczart addkey --location=/tmp/keys --status=primary
Then you can refer to the key in your code:
Crypter crypter = new Crypter("/tmp/keys");
String ciphertext = crypter.encrypt("Secret message");
If you already have some private key (X.509), you can import it.