Kbin syntax is @climbing@lemmy.ml, for starters. Same as Mastodon.

If you’re using the search in the magazines tab, leave off the prefix to the community name. So you’d want to search for climbing@lemmy.ml
And if you are the first person to search for a community on another instance, you won’t see any of the old posts because it is not retroactive, but any new content will show up as its posted