How do I install the latest build of xmllint on OSX 10.7.5
You can install fink then you can install a deb package from here: http://pdb.finkproject.org/pdb/package.php/libxml2-bin or compile one of the source packages.
You can learn more about fink such as how to install it Installing fink and Installing Packages
The name of the package should be libxml12-bin for the package install commands.
One thing to keep in mind is "The project offers precompiled binary packages as well as a fully automated build-from-source system. " so you can get the latest versions of utilities that are unix/linux/bsd in nature, and if there is no binary build, a few commands can let you build it from source.