It's limited to Public blogs right now (a paid feature) so we can slowly roll this out and plug holes as needed -- once I'm confident things are stable in the next few days, it'll be available to everyone for free.
You can read a bit more about the update, and I'd love to hear any feedback / ideas / bugs here! https://discuss.write.as/t/activitypub-support/64
In the web UI of Mastodon, you enter the address of the thing you're trying to follow in the search box, and you will get a result with an option to follow.
And I must say, that's pretty friggin cool. 😮 Thank-you Mastodon, ActivityPub, and write.as!
You'll notice this with new Mastodon accounts too -- Masto just doesn't fetch old posts. But once someone on your instance is following an account, you'll start to see the profile populated with posts