Probably the magazine has to be federated first - and so it has to be searched from the instance. I'll provide example using magazine AskKbin and assuming you're using browser version of Lemmy. The local adress for magazine is https://kbin.social/m/AskKbin
Go to your Lemmy and press search button. Search for the magazine using the local adress of the magazine (https://kbin.social/m/AskKbin). Once searched, it should start federating and you can subscribe to it. You should be able to easily subscribe to the magazine as well - as it is going to appear as community in search results. Similiarly you would initialte federation with communities from other instances of lemmy, if they aren't federated yet