One really bad and confusing thing about kbin is there are two searches. There is one at the Magazine list and another behind the magnifying glass.

The "magazines" only shows communities that someone else on this instance has already subscribed to. So if you search there for something mainstream then chances are you'll find it. But something niche from a small server, like theonion@midwest.social won't show up as no one from here has subscribed to it before so kbin.social doesn't know about it.

The magnifying glass search is the way to subscribe to a community that kbin doesn't know about yet. If you put in theonion@midwest.social into the search (NB, no ! on the start, like lemmy does it) then you'll be subscribed to it straight away and from then on it'll show up in the Community List for everyone.