You'll see a button appears on the top right corner of the timeline when you have new messages, with the number of new messages. You can press that to load them. Alternatively, you can go to the settings on the top right corner and change it so that it automatically loads new messages when you're scrolled to the top.

The instance has a "get started" link on the left that explains some stuff. And you can ask the instance admin kaniini for questions (who is also a Pleroma dev).