Destination toolbar disappeared on XCode 6.4
Looks like Xcode hides that menu when the window is a certain size. I have to make my window quite large before it comes back.
Not a fix as such but you can work around it using the menus: Product > Scheme and Product > Destination
You're probably running into the same issue I am. Like @BrandonWilliams said in his answer, it appears again if the Xcode window is wide enough. The underlying cause, for me at least, seems to be that in this build of Xcode (6.4) running on El Capitan beta 2 (with Xcode 7 beta installed), I am seeing duplicate simulators for iOS 8.4. And since there are two of the same version, the Schemes dropdown shows some sort of long GUID next to each one, causing the Scheme dropdown to be quite large:
