Xcode 6.4 Scheme Option Disappeared in El Capitan Beta
You don't need that button; the same options are available from the Product menu. So even though it's a bug, you can work around it for now, easily enough.
It is a UI bug and will probably be fixed soon. I've been working around the issue by dragging the Xcode window so that it is very wide (wider than my monitor). After a certain width, the scheme selector comes back into view.
Make xCode fullscreen. It works properly only in fullscreen mode.
UPDATED Method valid only for MBP 15 inch (retina)