If you installed XLingPaper before version 2.20.0 (October 3, 2012) and if you need to use Graphite capabilities with some fonts or if you need to hyphenate languages other than English or if you want to use the framed units capability or if you want to use mediaObject elements, then you either need to download and install the latest version of XLingPaper (see the Install page) or you need to get and install the XeLaTeX upgrade package.

To do the latter, download and install the package per your operating system:

Operating System Installer to download Size How to install
Windows XeLaTeXUpgradeE.exe 29MB Run the installer. It will do all that is needed.
Mac OS X XeLaTeXUpgradeE.dmg 45MB Run the installer. It will do all that is needed.
Linux XeLaTeXUpgradeE.tar.gz 42MB
  1. Decompress and extract the installer in some directory.
  2. In a Terminal, do 
    sudo ./installXeLaTeXUpgrade