Have you tried connecting the Nexus 7 to your Ubuntu machine using the USB cable instead? It will be much more reliable.
It works (for me) using Go-mtpfs with a direct usb connection.
To install, run the following commands in the terminal:
sudo add-apt-repository ppa:webupd8team/unstable
sudo apt-get update
sudo apt-get install go-mtpfs
You can also install the Unity launcher for it by running the following command:
sudo apt-get install go-mtpfs-unity
Before you try to mount the device, make sure it is unlocked (you'll know if it is or not if you see the open lock symbol at the bottom of the startup splash screen (underneath the Google logo) - if you don't see the symbol, it isn't unlocked).
Then connect your Nexus 7 via USB, and in the terminal type the following command:
fusermount -u /media/MyAndroid
This way has always worked for me, anyway. I was looking for the same information myself and I got it from here: http://www.webupd8.org/2012/12/how-to-mount-android-40-ubuntu-go-mtpfs.html
Give us some more info if you are still stuck... Which version of Ubuntu are you running, the vanilla 13.04 that you download from ubuntu.com? What steps have you done etc etc