I ended up having to email the corporate membership department and they had me create a whole new online account on brooksbrothers.com, and then re-sign up for corporate membership. It was a total pain but they did finally get it squared away for me so now when I log in, it says 'Welcome to Corporate Membership' each time.
I finally figured out how to create an account correctly for the corporate membership. For some reason the link to register an account from "how to participate" instructions brings you to a registration form which does NOT allow you to link your membership ID. Instead, you need to use the main site's link to register a new account.