A person I wish to contact, posted his public key as his contact info. I have setup Thunderbird and generated my on key pairs but I am missing something. I do not understand how to use just his public key to send him an encrypted email. Other than creating a message and choosing encrypted, what goes in the To: line? Where do I enter his public key? He did not list a key-ID or email address
There's and add-on for thunderbird called enigmail that will help you.
Install this add-on, then import the public key of your friend. When writing a new email, you will see that enigmail has added new buttons to thunderbird to write signed and/or encrypted mails.