There are two ways to install BrlTex now, either with the install script or manual install. NOTE: Other packages, such as python and plasTeX, are written by other authors and may have different licenses to BrlTex.
For this you will need python from www.python.org, and a copy of BrlTex from http://brltex.sf.net or the latest development version from SVN.
To install, go to the BrlTex directory, and run install.py, NOTE: you may need administrator/root permissions on some systems to do this. This will download all other components and install them, if it is successful it will inform you that you can then run BrlTex.py. After this, if you got BrlTex from SVN, you can update it just by running "svn update", you will not need to run install.py again. If it was unsuccessful, either you will need to try and work out what went wrong or follow the manual install instructions
You will need python from www.python.org, plasTeX from http://plastex.sourceforge.net and liblouis from www.jjb-software.com, as well as a copy of BrlTex from http://brltex.sf.net or the latest development version from SVN. There is also a copy of liblouis at the BrlTex website which includes a precompiled windows binary.
Follow the install instructions for python. Now decompress plasTeX, and modify the file Entities.py in plasTeX/Base/LaTeX/. You need to comment out the last line of Entities.py, by putting a # at the beginning of the line. Now install plasTeX as usual. If you are using Windows and have liblouis from the BrlTex website, you can copy liblouis.dll from that into the liblouis directory in the BrlTex directory, otherwise compile liblouis for your platform, and move the created dynamic library (shared library) into the liblouis directory in the BrlTex directory. Now you should be able to run BrlTex.py.