In Xcode 6 Beta 5 and previous betas, I could open a HomeKit Accessory Simulator by right-clicking on the icon in the dock and choosing Open Developer Tool > HomeKit Accessory Simulator. In Xcode 6 beta 6, this seems to be no longer the case:
How can I launch the accessory simulator now?
if you read release notes
HomeKit Accessory Simulator
• HomeKit Accessory Simulator is now included in a separate product called Hardware IO Tools for Xcode, available on developer.apple.com/downloads. (17738621)