Zbar and imagemagick need to be installed which usually can be found in the repositories or on their websites http://zbar.sourceforge.net/ and http://www.imagemagick.org/
Make a photo of the qr-code with a program like cheese. Open the photo with a program like shotwell. Open a terminal an type in:
import :- | zbarimg :-
A cross-hair will appear with which you can select the qr-code. As a result an url/bitcoinaddress will be returned.