ok, I could solve the problem now. If anybody else is interested, I write how it´s done:
Within the arduino program the file RXTXcomm.jar file which you have to replace with the one from processing is found here:
right-click on arduino 03.app and choose "show packet contents". Then goto contents/ressources/java/
There you´ll find the RXTXcomm.jar file.
With now both the RXTXcomm.jar file and the ibrxtxSerial.jnilib file having replaced with the files found in processing, it all works well, it seems. (as nick has stated before)
Regards,
Helge