Often users will call in presenting a few different issues, all of which stem from needing to register their computer.
Outlook will throw a couple of of different errors, usually in regards to an invalid security certificate on autodiscover.exchange.tufts.edu.
Firefox, Safari and Chrome get thrown off by tufts forwarding folks to portal.net.tufts.edu, they seem to think it may be something malicious. It should be fine to either confirm security exceptions or just to go directly to portal.net.tufts.edu.
Now that you've gotten to the site, it should be fairly straightforward, it even has the ability to detect MAC addresses. Here are some screenshots in case you're walking someone through it. The one thing to note is after registration it takes up to 15 minutes to process through the system, and in that time period there's no clear indication that registration worked beyond waiting!