For many people, the normal Installing RepRap on your computer instructions work great.
However, The standard jars and .so files that ship with the reprap host application appear to be incompatible with Ubuntu 11.04. This doesn't quite make sense to me as yet I will do some more research when I'm a little less tired. In the mean time here is the fix:
install j3dcore under Ubuntu 11.04 x64
Determine your java install directory;
You will likely have to chase down the real directory as the above will be a symlink.
My final directory was:
Start by deleting the following files (if the exist):
lib/ext/j3dcore.jar lib/ext/j3dutils.jar lib/ext/vecmath.jar
Now download the latest version of the 3d libraries from:
I used the 1.5.2 versions.
For Ubuntu x64 you need to download:
Extract the files from the zip. (you may find an internal zip so extract that as well).
Now move all jars contained in the zip to the 'lib' directory under your reprap host install
Now move the .so file into the machine specific directory
You should now be able to start reprap.
You can also install these files so they are available globally (handy for debugging). In this case you need to install the jars into the jre's lib/ext directory and the .so into the lib/amd64 directory.
sudo cp libj3dcore-ogl.so /usr/lib/jvm/java-6-openjdk/jre/lib/amd64/libj3dcore-ogl.so
sudo cp j3dcore.jar /usr/lib/jvm/java-6-openjdk/jre/lib/ext/ sudo cp j3dutils.jar /usr/lib/jvm/java-6-openjdk/jre/lib/ext/ sudo cp vecmath.jar /usr/lib/jvm/java-6-openjdk/jre/lib/ext/