Go to the magazine that you want to create a thread in, hit the + in the top bar, and select "Add new link" or "Add new thread". There's also a select box for the magazine once you hit one of those.

For a microblog post, make sure to hit "Add new post" and again, there is a dropdown select box for the magazine. Also, you can add a hashtag to make the microblog post show up in various magazines.

There is a plus up in the top right group of icons. When you click it, it gives you options to choose from and asks if you want to make a thread, post, link,ect. Once you are in that page there is an option for what magazine you want it to go in. [FYI I have been using kbin for three days so I could be wrong.]