You need to configure a jBPM runtime (point it to a folder containing the right version of the jBPM libraries you want to use) first.
I would recommend using the installer script to generate a fully preconfigured eclipse environment.
If you want to manually do this: - unzip the -bin zip from jbpm-installer/lib to a folder - in eclipse preferences, under jBPM, create a new jBPM runtime and point to the folder you just created - restart eclipse for the changes to take effect