Everything (the header file, examples, documentation and all other things referred to on this page) is contained in the tarball
Feel free to copy the header tree.hh (which is all you need code-wise) into your own source directory as long as you respect the license (see above).
The list of changes can be found in the ChangeLog.
There is a small utility library tree_util.hh originally written by Linda Buisman. This library contains a number of functions to output a tree as text, to aid in debugging your project.
The current version works with GNU gcc 3.x and higher, Borland C++ builder and Microsoft Visual C++ 7.1 and higher (I no longer support older versions of Visual C++). It is compatible with STLport (though older versions may not work correctly, best to upgrade to something fairly recent).
The library is available for free, but if it helps you a lot, why not consider a small donation? Your contribution is greatly appreciated!
No installation is required as the tree.hh library is contained in the single header file tree.hh. Just copy it into your source directory (and upgrade it from the master site when a new version becomes available).