Just to make clear: if you want to just use Activiti, there is no need to compile and package Activiti yourself. You can just download the zip file from activiti.org, it contains everything you need.
If you do want to compile it yourself: the error you are getting is a known bug in the Xalan XML parser.
If I'm correct, the logging output just before that error must have displayed something like 'if you get an error, first do this'. You need to execute another ant target (the exact name is in the logging message), which will patch Xalan, and all will run smooth thenm.
