You can only access it if you get the email.
There is a link that takes you to a page with the offer.
I was on a connecting flight. The first flight had no gate announcement about the free checked bag, but the second flight did.
Obviously, Frontier was trying to head off a lot of carry on bags by making the offer early for the second flight.
Btw, has anyone noticed how many people are in Zone 1 now (that have carry on bags)?