Well fortunately there's an easy solution to this little annoyance that involves the little button that's come to be known as the "lozenge." It's that little gem-like button on the top right of OS X windows, and what it does is reveal or hide toolbars. If you click on it in the Finder it will show or hide your tool- and sidebars. So in my case here, if I just click the lozenge I can see my sidebar again (see picture after the jump) and it's easy to install the application. The lozenge works the same way in other applications, and you can use it to hide toolbars when you want more screen real estate for your window content. If you command-click the lozenge it will cycle through the toolbar options.