Mendix does not currently ship with a default refresh button. The silver lining is that, in most cases, you don't need one! Mendix recently implemented static URLs, which means you can simply use the browser's in-built refresh behavior (aka f5).
Where Mendix used to redirect to the home page on f5, it will now attempt to reload the current page, assuming it has a configured URL. So as long as the URL property of your page is set, you should be fine.
Of course, if you absolutely require a refresh button, you could always build a show-page action button that opens the current page. Not exactly scalable, but it gets the job done.
Refresh button is back (at least available in mendix 6.10 as widget button)