PGP question

Im doing this assignment for one of my security classes and I can't figure out this stupid program. Our prof wants us to send him a message that is encrypted with his public key. I see the integrated pgp options on the outlook toolbar, and I can select encrypt message, but how do I select the key I want to use to encrypt with?? I keep going to PGPkey and that doesnt seem to allow you to select the key you want, anyone know how to do this???? Plz help me!

Not sure with Outlook as I've only used GPG on linux, but you will want to import the key, the key is chosen by the email address in the public key, at least that's how it was on linux.