Thanks for the help, Mod. Looks like user error on my part. For anyone else who stumbles upon this:
When you create an album in Photopost and select to make it a private album, you're assigned a password on the next page. I missed seeing that password when I created the album. You can always see the password again or generate a new password if you go to your "My Albums" page and then edit that album. The only catch seems to be that you can't pick the password; one is generated for you.
Also, your Friends can see private albums. So don't put up any naked NSX photos unless you're ready to share them with your Friends. More info here:
http://www.nsxprime.com/forum/faq.php?faq=vb3_user_profile#faq_vb3_friends_contacts
- Public albums can be viewed by anyone
- Private albums can only be viewed by site staff (moderators, administrators) and your Friends and Contacts (info)
- Profile albums are viewable only by you. However, you can use the images to customize your public profile (info)