Click on the white +1 button. It will turn blue and a bubble will appear (shown in next picture). If it disappears, move the mouse and then put it on top of the now blue button again.
When you see the little bubble come out of the +1 button, click on the textbox in it.
In the lower part of the snippet, right above the "Share" button, there is a textbox that allows you to choose who you're sharing this with. Please click on it
In the list, select 'Public'
To complete your post, click 'Share'