After upgrade to lollipop, LG D415 grays out "Enable USB Debugging" when phone is connected to PC
I had to connect as a camera (i.e. pick "PTP" from the item in the drop-down menu option you get when a phone is connected) to get the device to show up in Device Manager (it showed up near the top under Android Devices).  From that point, it was standard: right click on the entry under Android Devices, select "Update Driver Software", click "Let me pick from a list of device drivers on my computer", then pick "Android ADB Interface" under "Google, Inc." as the manufacturer.

I had previously tried (several times) connecting charge-only and "MTP" and the device didn't show up under Device Manager.  It looks like I always need to connect as PTP; MTP and "debug only" still leave my device invisible to adb devices.