Which Version of eclipse are you using ? GPD works for me with Eclipse 3.4.1(Ganymede). If you are using 3.4.1 look whether your eclipse has all the dependencies installed.
I use jBoss 4.2.3 to run my jBPM 3.3.1 (I had tried to use JBoss 5 but was not successful). To get started with jBPM 3.3.1 installation download the jbpm-installer-3.3.1.GA.jar file and key in
java -jar jbpm-installer-3.3.1.GA.jar and follow the instructions of the installer.
Once installed start JBOSS and access the following link