Before the Ubuntu upgrade
Before upgrading, check if all your non-Ubuntu repository applications are available. For XBMC you can check the XBMC roadmap
After the Ubuntu upgrade
We need to reinstall all applications, which are not in the Ubuntu repository. They were cleaned up automatically.
See here for all instructions: Installing XBMC on Ubuntu
Download the debian package here: dropbox.com/downloading Linux
Just open the downloaded Debian package (Ubuntu x86 .deb).