You can use https://browse.feddit.de/ to find a community, then copy the address (https://lemmy.ml/c/jerboa for example) into your instance's search bar, give it time to fetch it (it will show "no results found" at first, be patient), then click on the link that pops in search, open the sidebar from there and subscribe.

Yeah, it's super awkward now, but it's all growing pains - it's only going to get better from here!