Where do I enter my private key to generate an address off of ?
You don't. Each new address also has a new private key. Start with a simple example to see you have done it right,
For CPU: vanitygen64 1Love
For GPU: oclvanitygen -d 0 1Love
That should spit out an address/key pair in a few seconds typically. If it works then you can try longer ones and other options.
You only supply a "hex public key" as input when using -P to generate a partial key for a third party.