You can go to the member site and on the "Vacation Planning Tab" select Disney's Magical Express.
On the bottom of that page is a link to the page to sign up for the Magical Express or update your flight information, etc. Just fill out that page and stop worrying. Since you put in your reservation number they will be able to tell if it is a duplicate, but you will not need to worry or bother Member Services. If you get two packets instead of one it won't matter either.