The developer version releases available from 

  http://www.cs.waikato.ac.nz/ml/weka/index_downloading.html

are all compiled for Java 1.6.

However, for Mac OS X Java 1.6 is only available for 64 bits, and in
particular the CRAN Mac OS X build/check system is 32 bits.

Hence, recompile weka.jar for Java 1.5.

To do so, go to ~/src/org/weka/weka.

Ensure that build.xml has

  source="1.5" target="1.5"

in its compile target.

Then do

  ant compile
  ant exejar

and use dist/weka.jar for RWekajars/inst/java/weka.jar.  
