How can I install the new virtual machine app Boxes (https://live.gnome.org/Boxes) in Ubuntu?
You can't install the GNOME boxes in 11.10 unless you don't mind risking a complete breakage of your system. This is because GNOME boxes depends on components that are part of GNOME 3.3+, which are not in the Oneiric's repositories. What this means is that you have to try and backport all these components for 11.10, which will invariably break other components that are dependant on earlier GNOME components. To cut short, I would advise against doing so.
If in case you change your mind, you can install it in a 12.04 system using a PPA.
Note that this PPA only has 3.3.5 as of writing. If you are looking to use a newer version, you can consider installing it from the source tarballs.
Do take note that the GNOME boxes is still very much in development.