I think the USNA portal only opens up if you're accepted to NASS. I was rejected from NASS, and I found that out through email. I was able to open a portal once I was deemed an official candidate for admission(March, 2015), and that is the same portal that is used for the rest of the admissions process. You can sign into this portal(called the CIS) using info that USNA provides with the candidate package. Since NASS acceptance means you are automatically an official candidate, I would assume this is when you can view the portal.