Dear Aunt TUAW,
I miss the lozenge. You know what I mean? The chicklet thing that used to be at the top right of my Finder windows. Click it and it would show the sidebar. Click again, sidebar gone. How can I show and hide the sidebar in Lion?
Your loving nephew,
You refer to, of course, the savior of everyone who used to struggle with DMGs on Leopard and Snow Leopard. Auntie remembers the lozenge fondly.
Unfortunately, the lozenge is a thing of the past with OS X 10.7 Lion. But that doesn't mean you can't still hide and show your sidebars. Just right-click on the title bar of any Finder Window and select Show Toolbar or Hide Toolbar from the popup. Unfortunately this affects both the toolbar and the sidebar, but Auntie takes what she can get.
Hopes this helps next time you run across one of those unfriendly windows.