Just FYI, the “All” tab only lists the communities that the instance you are on is subscribed to. You can get a larger searchable list from https://browse.feddit.de/ although it only seems to be updated once a day or so.

If you find something that interests you on the feddit search page, you can manually search for and add that community to the instance your account is on.

Just copy the link to the community from the search page and paste it into the search box on your instance. You may also need to change the top dropdown on the Search page from Communities to All, depending on if someone else on your instance has previously searched for it.

You should then see the community listed in the results as something like Community Name - X subscribers. Click that link, then click subscribe on the top right box on the next page.