Linux won't talk to my phone by USB. Ideas?

I'm following along with "Building a Simple Android App" at https://teamtreehouse.com/library/build-a-simple-android-app/testing-and-debugging/running-the-app-on-a-device

and although I want to install the app to my phone, there's no evidence that my computer is aware of my phone, other than the fact that it's charging the phone.

Checklist of things done:

• On phone, enabled developer options by clicking version number 7 times.
• On phone, enable USB debugging
• The "ADB Logs" tab in Android Studio is empty

Reading from "Using Hardware Devices" at http://developer.android.com/tools/device.html, subsection "Setting up a Device for Development"

• Added android:debuggable="true" to the project's AndroidManifest.xml file and rebuilt
• Set /etc/udev/rules.d/51-android.rules to contain just the line SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", MODE="0666", GROUP="plugdev" (I think this is the proper text for a Google Nexus 5, the above link was a little ambiguous IMO).
• /etc/udev/rules.d/51-android.rules is root.root -rw-r--r--
• Did not actually reboot
• Did plug in & out the phone several times.

When I run "adb devices", the utility returns an empty list. When I use the IDE's Android DDMS tab thing, in the "Devices | logcat" tab it declares No Connected Devices. There's a GUI app called "Android Device Manager" that can be launched from the IDE. It also has an empty list of devices.

I looked around in /var/log/ for anything interesting - found nothing. No USB announcements of any sort when the thing is plugged in our taken out. That seems wrong.

If the Minds here have no suggestions, I guess I'm going to try to get MTP file transfer going between the phone and the computer. Maybe that'll shake up the USB system enough for something to start working....? :(