The following tutorial assumes you are running Mac OS X Leopard. Leopard is needed to make x64 builds, compatible with the new Java 6 from Apple.
Before you can compile lwjgl, please check it out from SVN by issuing the following command: svn co https://java-game-lib.svn.sourceforge.net/svnroot/java-game-lib/trunk/LWJGL lwjgl
Then enter the lwjgl directory and issue the following commands:
The output should be placed in libs/macosx and contain a fat binary with support for 32bit (x86 & ppc) and 64bit (x86-64).