I am not sure how to see the main feed of mastodon from kbin but you can use https://kbin.social/u/@USERNAME@INSTANCENAME to view posts from a specific user.
Kbin seems to treat individual accounts on mastodon as different magazines.
Also from Mastodon it will not appear in the main feed but you can use a fediverse link to view it and comment on it.
Next to comments and posts where it says more it will give you a fediverse link. If someone puts that in the search bar in mastodon they can connect to it.