If nobody has ever subscribed to a foreign instance's community/magazine before, it won't show up on your home instance. Currently, the best way to pull it into your local instance is to copy its web address on the other site into your local search.
e.g. If you wanted to pull in kbin.social's AskKbin from lemmy.world you'd find its URL, https://kbin.social/m/AskKbin, and paste that address into your home instance's search box. As long as somebody has done this once, AskKbin will now show up in regular community lists, searches, etc.
bananaghost