set up device for development (???????????? no permissions)

I am using a Samsung galaxy nexus phone (Android 4.0 platform) .

I am developing Android app on Ubuntu linux OS. I would like to run my application directly on the Samsung handset device, so I performed the following setup steps:

1. in my project AndroidManifest.xml file, added android:debuggable="true" to the <application> element

2. On the device, in the Settings > Security enabled Unknown sources

3. On the device, in the Settings > Developer options enabled USB debugging

4. On my computer, created the /etc/udev/rules.d/51-android.rules file with the following content:

   SUBSYSTEM=="usb", ATTR{idVendor}=="04E8", MODE="0666", GROUP="plugdev" 
   

5. On my computer, run the chmod a+r /etc/udev/rules.d/51-android.rules command

Then, on my computer I opened a terminal and executed the adb devices command, I got:

List of devices attached 
????????????    no permissions

Since I did not see my device but only ???????????? no permissions, I then run the following commands:

 adb kill-server
 adb start-server
 adb devices

But I still got:

List of devices attached 
????????????    no permissions

Why? What am I missing?