I recently bought LG G3 and I have enabled debugging mode but Eclipse is still not recognizing it, back when I used the Galaxy S3 for development I had to install Samsung Ki
If you have this problem on Ubuntu; here is what I did to get it working:
Switch to PTP from MTP on the phone (this assumes you're connected via USB)
Disconnect the USB cable
Go to settings -> developer options and select revoke usb authorizations then ok
Make sure USB Debugging is checked
Reconnect the USB cable and your phone will say its connected as an installer; wait a minute or three and debugging mode should appear on the status bar (on the phone)
Open Eclipse and go to DDMS
Select "reset adb" in the dropdown if your phone isn't appearing yet