Where did you get the jBPM-3.0.3?
I was getting that error when i was using the wrong version of the jar. Inside of the BpelExtension.zip there should be a jBPM-3.0.1 or jBPM-3.1? Jar.
I am using the beta version of jbpm-bpel so i am not sure of the exact version number.
If that is not the case i would also check the paths in your ant script and make sure that the right jbpm jar is first in the class path.
I also had this problem.You just need put the that file inside root path of the process archive that problem is resolved.