Ok, so it appears to be something weird about the interaction between Lemmy and kbin. The !magazinename@instance.tld seems not to do the job. However, if you copy the url to one of the submissions to the magazine in to the search box here, that will pull across the submission, and incidnetally let you join the magazine the submission was made to

Search for [!firefox@fedia.io](/c/firefox@fedia.io) in the search box. You might need to do it a couple of times with 30 seconds or so between each search. Group discovery isn't exactly intuitive in lemmy yet if you're the first person to use a particular group on your instance