There are two ways of searching for a magazine:
Option 1: Go to the list of magazines and search there. Downside is that you will only find communities/magazines your kbin instance knows about (at least one person on your instance is subscribed to it).
Option 2: Enter the full name of the community/magazine into the general search ("newcommunities@lemmy.world" without the quotation marks, for example). This lets you subscribe to communities your instance didn't know about already. To find these communities/magazines your instance doesn't already know about, use something like lemmyverse (has a separate search for kbin magazines)