Here are some brief instructions to install the Newton physics engine
in OpenSim.

1. Download the source for OpenSim, for example from their SVN server:

  svn co http://opensimulator.org/svn/opensim/trunk opensim

You will need the open-source .NET framework mono (see
http://www.mono-project.com/Main_Page ) to build OpenSim.

2. Get the NewtonPhysics engine from its SVN repository:

  svn checkout http://forge.opensimulator.org/svn/micasim/trunk micasim

3. Patch the prebuild.xml file in the OpenSim root directory with the
patch file in the micasim root directory:

  cd opensim
  patch -p0 < ../micasim/prebuild.patch

4. Copy the NewtonPlugin directory in the micasim root directory to
opensim/OpenSim/Region/Physics/ .

5. Edit the OpenSim configuration file (found in
<opensim-root>/bin/OpenSim.ini) to change the "physics = basicphysics"
to "physics = Newton".

6. Build OpenSim (follow their instructions---as of 31 Oct 2008, this
means run ./runprebuild.sh, then run nant).

7. Start an OpenSim server: mono opensim/bin/OpenSim.exe .

8. Connect to that server with your SecondLife client (here is the
command line to type on my Mac OS X box):

  /Applications/Second\ Life.app/Contents/MacOS/Second\ Life -loginuri http://127.0.0.1:9000/  

9. Rez prims, and make them physical.  All physical prims evolve
according to Newtonian gravity.

10. Enjoy.  If you make a change to the micasim code, you need only
repeat steps 3, 4, and 6, and then you're ready to go with the updated
physics engine.