The problems seems to be the jar container created by the plugin in the project. In 3.3M5 it seems to only have the JDT. So for example classes like RuleBase and WorkingMemory are not found. This not really a problem for me since I'm going to add the jars to the project directly (as opposed to using the eclipse plugin.) But I'm not sure yet whether the compiler or debugger will work with 3.3M5.