Using Andrioid Studio (Preview 0.4.3) on my MacBook Pro running OSX 10.9.1, when I tried to run an Android app on an LG G2 Andriod phone connected via USB, I either didn't see the device appear at the Choose Device dialogue at all or saw the device but with an offline state. Here are the hacks that got me over these problems. In particular, the part on enabling the Developer Settings on an Android phone seems particularly hacky.

1. On the phone, enable USB Debugging from Developer Settings, which is hidden by default on Android 4.2+ devices like LG G2. To enable USB Debugging, go to

   Settings -> About Phone -> Software Information

   and tap the Build Number seven times. You should now see a new Developer Settings sub-menu inside Settings, which contains the option to enable USB Debugging.
2. On my mac, restart adb by issuing 'adb kill-server' followed by 'adb start-server' at a cmd prompt; The adb command can be found under /Applications/Android Studio.app/sdk/platform-tools
3. Disable and re-enable USB Debugging on the phone
4. Rebooting the phone if it still doesn't work.

(Finally succeeded after many attempts, switching USB cable and ports also; plus answering a prompt on the phone on accepting a fingerprint when my mac tried to connect to it.)

Good luck!

Sources:

• Stack Overflow
• Androidbeat