Not sure what you did, but I just downloaded Eclipse Helios 3.6 RC4, Eclipse IDE for Java Developers, and then installed from the update site at: http://download.jboss.org/drools/release/5.5.0.Final/org.drools.updatesite/
After restarting Eclipse at the end of the installation, I could open the Drools and jBPM perspectives without any issues.
I'd also recommend to use a more recent version of Eclipse though.
Kris