I know it's been a while since this question was asked, but I was having similar problems (albeit with 14.04) and thought I'd post my solution.
I also had QGIS 1.8 installed (and later again tried this with 2.18 to no avail), and the solution given by jbalk did not work because this option didn't exist in the drop-down list. While the OpenGDB driver is installed in QGIS by default, it is only the case when it is compiled against GDAL version 1.11.0 or higher (see RyanDalton's comment on File Geodatabase (*.gdb) support in QGIS?). When installing the stable version of QGIS version according to the QGIS website instructions, I ended up with a GDAL version of 0.9.#.
You can check your GDAL version by typing
gdalinfo --version
The easiest way to fix this now (perhaps this update didn't exist back in 2013) is to install using the ubuntugis-based repositories according to the instructions on the QGIS website. The GDAL version will then be up-to-date (mine currently reads 2.1.0), the OpenGDB driver will work out of the box, and you can follow jbalk's instructions.
So I tried some more things and came up with a solution. I will go over what I did step by step and hopefully this will work for others.
First thing I did was uninstall qgis as completly as I know how
sudo apt-get autoremove qgis
sudo apt-get autoclean
Then I removed the Ubuntugis-unstable PPA
sudo apt-add-repository remove PPA:ubuntugis/ubuntugis-unstable
I went to the qgis website and added the qgis Dufor deb and deb-src to my software & updates application
deb http://qgis.org/debian raring main
deb-src http://qgis.org/debian raring main
Next I added the keyserver info by typeing the following commands into the terminal
gpg --keyserver keyserver.ubuntu.com --recv 47765B75
gpg --export --armor 47765B75 | sudo apt-key add -
Once I had everything added to my list of repositories I used the update command
sudo apt-get update
From here I ran 4 commands to install qgis and saga. I had to do each command separately, whenever I tried to do them all in one go I would get error messages
sudo apt-get install python-qgis-common
sudo apt-get install python-qgis
sudo apt-get install qgis
sudo apt-get install saga
Once all of this had been completed I was able to run qgis and saga at the same time and sextante was able to find and execute saga modules.
Best Answer
Try the following commands from a command line:
The problem may be that synaptic has not updated it sources because the signing key was not imported. This should be the most easy way to fix that error.
[update] It seems the wxwidgets package for raring was uploaded but not build for some reason. This is now fixed. You should be able to install saga on raring.