Yes, you need an Apple developer account. Using that, you generate a certificate and a provisioning profile. I don't know the details, we have an Enterprise account and I have a co-worker who sorted out the credentials.
So far, I'm not impressed with the Captivate App Packager. Using the PhoneGap Build website is easier. Packager seems to be a low-usability, fragile wrapper around the website.
-- Hal
