As this is still a new server the communities you may want to join may not yet be shown on unilem.org. Use a site like lemmyverse to find your favourite communities. Copy the community id eg. !lemmy@lemmy.ml and paste in the search. Wait a few seconds and the community will pop up and you can subscribe. If you still need help DM me or reply in the comments.
Interesting update: after posting this, I was able to click on them like links and then subscribe. Still what am I doing wrong when I paste those in the search?