Did you try from your command line (assumed linux)
adb devices
If you got a lot of question marks
List of devices attached
?????????????? device
Check what is the vendor ID of your device:
lsusb
> Bus 003 Device 006: ID 04e8:689e Samsung Electronics Co., Ltd
Vendor ID is ’04e8′ in this case.
You need to add your device to USB list
sudo vim /etc/udev/rules.d/51-android.rules
and put this line (use your own vendor ID).
SUBSYSTEM=="usb", SYSFS{idVendor}=="04e8", MODE="0666"
Alternatively try the following syntax as pointed out by mcordaro and Yekmer Simsek referring to the documentation
SUBSYSTEM=="usb", ATTR{idVendor}=="04e8", MODE="0666", GROUP="plugdev"
save it, and then
sudo chmod a+r /etc/udev/rules.d/51-android.rules
sudo service udev reload
and it should work now.