Kbin/Mbin handle microblog posts kinda oddly. They're not actually attached to the magazine you see them in, in most cases, and it's instead *bin auto-sorting posts by hashtag. *bin will aggregate all of the microblog content and sort it by the first hashtag listed, and that'll determine which local magazine that post will appear in.
So for instance, I could post:
Just got the high score! #gaming
And this will show up in /m/gaming for you, even though I didn't specify any magazine to post it to. I could do that from my Mastodon account, even, and you'd see it there, as well. You can subscribe to local magazines to effectively subscribe to Mastodon hashtags this way.
Posting microblog content directly to a magazine is a bit weird, as *bin basically just adds a "hidden" hashtag with the magazine's full address in the metadata. So if you wanted to post a microblog post to this magazine, for instance, you could do that by either selecting this magazine from the list when posting, or you could include #fediverse@lemmy.world as a hashtag from any Fediverse platform, and it'll show up in the microblog tab on *bin.
I hope that makes sense; I might not be explaining it properly lol