At the moment the device simulator does not work on MAC OS Yosemite. We are investigating the cause for the issue and the task is logged with the highest priority in our internal issue tracker.
However, I cannot yet commit to a specific time frame for a fix. Please accept our apology for this inconvenience. I hope that until we address the issue, you will be able to test that last issues in your app either in a native emulator
or in the In-Browser AppBuilder