Xeiso requires Pyglet 1.1 to run. If you installed Xeiso from the Launchpad PPA, you already have it. To install:

wget http://pyglet.googlecode.com/files/pyglet-1.1.tar.gz
tar xvzf pyglet-1.1.tar.gz
cd pyglet-1.1
sudo python setup.py install

Once Pyglet has been successfully installed, you may continue installing Xeiso.