If it's the first time searched from your instance, it takes time to find. Couple that with the server load makes it even longer.
Search using the search function and put in a community like [email protected], that or the whole url, and wait till it appears. You can go back to search and search it again after doing something else.
If you don't have an account it seems it just uses lemmy.ml by default in Jerboa. If you want to use a different instance, you need to signup and then login.
For adding content from other instances, you can likely find it with https://browse.feddit.de/ (the built in communities search shows some but not all instances). Then e.g. if you're using lemmy.ml adding cricket community from aussie.zone:
Interesting. I'm on lemmy.ml using Jeroba, when I go into the Canberra community in the app I can only see one post and one comment (my own), but when I look at the website there's four other comments. Any idea why that is? Is this the fault of the app? Or does it come down to the federated nature of Lemmy?
Mine is having the same issue. I checked with some other communities and it didn't seem to have that error. Can you check another instance and see if it's the same?