To post text, tap on the plus button in the center of the tab bar at the bottom of the screen. When the Composer appears, tap on the Text tab at the bottom of the screen. Here you will see a blank field where you can write text.
If you want to leave a reply:
As a reminder, if you want to reply to or leave a comment on someone else's update, the best way to do that is to tap on their update in the main feed. The keyboard will immediately appear and you can write your reply. It will appear directly below the update and be clearly attached to the content that you are replying to.