Hi stellina ,
I've suffered from the install.demo ant script , i advice you to do the download and installation manually
1) download jbpm files from this , http://hudson.jboss.org/hudson/job/jBPM5/lastSuccessfulBuild/artifact/target/
1) any version of eclipse (Personnaly i'm using helios) .
then follow the link post from Marie , but instead to add the plugins manually use help --> instal new software to add the jbpm plugin and drools tools that you've dowloaded in step 1 ,
and then it 'll works .