Hi Robot – I believe the theme developers hide the button to avoid having users post blank updates. If it is visible, and clickable, users might think that they should click it in order to post an update, when in reality, they need to enter something in the text area and then click the “Post Update” button (once it is visible) to finalize things.
I took a look at the CSS to see if it might be easy to do what you want, but it doesn’t seem all that straightforward. You might want to take a look yourself, if you still find that is what you want.