There are two files under jbpm-installer\db\driver
Am I missing more files?.
This question is 2 years old, but other people like me may still run across it. Here's a fix/solution:
Go to link below, and download the zip for the version that you already see in the jBPM distro you have. Likely 1.7.2.
Look in the zip file for the file named "slf4j-simple-1.7.2.jar", and add that as an external jar in your project build path (classpath)