[Nexus 7 Q&A, Help & Troubleshooting] Platinum1986:
Thanks again for your help.

So i typed this in the terminal:

sudo gedit /etc/udev/rules.d/51-android.rules

then i copied/pasted this:

# adb protocol on passion (Nexus One)
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="4e12", MODE="0600", OWNER="mikael"
# fastboot protocol on passion (Nexus One)
SUBSYSTEM=="usb", ATTR{idVendor}=="0bb4", ATTR{idProduct}=="0fff", MODE="0600", OWNER="mikael"
# adb protocol on crespo/crespo4g (Nexus S)
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="4e22", MODE="0600", OWNER="mikael"
# fastboot protocol on crespo/crespo4g (Nexus S)
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="4e20", MODE="0600", OWNER="mikael"
# adb protocol on stingray/wingray (Xoom)
SUBSYSTEM=="usb", ATTR{idVendor}=="22b8", ATTR{idProduct}=="70a9", MODE="0600", OWNER="mikael"
# fastboot protocol on stingray/wingray (Xoom)
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="708c", MODE="0600", OWNER="mikael"
# adb protocol on maguro/toro (Galaxy Nexus)
SUBSYSTEM=="usb", ATTR{idVendor}=="04e8", ATTR{idProduct}=="6860", MODE="0600", OWNER="mikael"
# fastboot protocol on maguro/toro (Galaxy Nexus)
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="4e30", MODE="0600", OWNER="mikael"
# adb protocol on panda (PandaBoard)
SUBSYSTEM=="usb", ATTR{idVendor}=="0451", ATTR{idProduct}=="d101", MODE="0600", OWNER="mikael"
# adb protocol on panda (PandaBoard ES)
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="d002", MODE="0600", OWNER="mikael"
# fastboot protocol on panda (PandaBoard)
SUBSYSTEM=="usb", ATTR{idVendor}=="0451", ATTR{idProduct}=="d022", MODE="0600", OWNER="mikael"
# usbboot protocol on panda (PandaBoard)
SUBSYSTEM=="usb", ATTR{idVendor}=="0451", ATTR{idProduct}=="d00f", MODE="0600", OWNER="mikael"
# usbboot protocol on panda (PandaBoard ES)
SUBSYSTEM=="usb", ATTR{idVendor}=="0451", ATTR{idProduct}=="d010", MODE="0600", OWNER="mikael"
# adb protocol on grouper/tilapia (Nexus 7)
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="4e42", MODE="0600", OWNER="mikael"
# fastboot protocol on grouper/tilapia (Nexus 7)
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="4e40", MODE="0600", OWNER="mikael"
# adb protocol on manta (Nexus 10)
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="4ee2", MODE="0600", OWNER="mikael"
# fastboot protocol on manta (Nexus 10)
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="4ee0", MODE="0600", OWNER="mikael"

##########################################################################################
# my own devices
##########################################################################################

# Google Nexus 7 16 Gb Bootloader & recovery mode
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="4e40", MODE="0666", OWNER="mikael" # Bootloader
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="d001", MODE="0666", OWNER="mikael" # Recovery

# Google Nexus 7 16 Gb PTP mode (camera)
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="4e43", MODE="0666", OWNER="mikael" # PTP media
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="4e44", MODE="0666", OWNER="mikael" # PTP media with USB debug on

# Google Nexus 7 16 Gb MTP mode (multimedia device)
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="4e41", MODE="0666", OWNER="mikael" # MTP media
SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", ATTR{idProduct}=="4e42", MODE="0666", OWNER="mikael" # MTP media with USB debug on

# Google Nexus 7 MTP mode : automatic unmount when unplugged
ENV{ID_MODEL}=="Nexus", ENV{ID_MODEL_ID}=="4e41", ACTION=="remove", RUN+="/usr/bin/fusermount -u /media/nexus7"
ENV{ID_MODEL}=="Nexus", ENV{ID_MODEL_ID}=="4e42", ACTION=="remove", RUN+="/usr/bin/fusermount -u /media/nexus7"

Is it ok?

I saved in gedit and quit, then i connected the Nexus 7 via USB and i tried this :

lsusb

Sadly the nexus 7 doesn't appear. :/

I'm on ubuntu 14.04 LTS.