Yes, the Candidate Portal is updated when an offer is extended. For me though (got my appt two weeks ago), the portal wasn't updated until the day I got my appointment in the mail, so I didn't get an early heads-up from the USMA website.
The appointment update will appear under the "nominations received" section of the overview tab. It includes a place to accept the offer and some forms that need to be filled out.
Hope that helps!