Some information here may be outdated as there was a recent update to the Lemmy server that's in the process of trickling down to instances and it may have fixed some of the glitches, but the general procedure still applies - i.e. paste the fully qualified community name in the search box and submit - it should eventually show up as a single result. Clicking that result will take you to the mirror of that federated community on your instance, on which you can then subscribe.