Just found a solution: http://blog.iharder.net/wp-content/uploads/2009/08/librxtxSerial.jnilib Download this file and put it in this directory library/java/extensions deleting the existing one if i understood well is a problem of libraries compatibility between 32 and 64 bit architectures
Check that all DLL functions referred in your Java code are correctly defined and exported and export the same datatypes as those your Java code is expecting. This won't stop the DLL loading, but it may well mess up the next stage - resolving link addresses or making functions that should work, fail in unexpected ways. Place the DLL in the current directory, in one of the directories listed in the PATH environment variable, or, best of all, in the native library search path, set using the system property java.library.path: