I would recommend to use RuntimeManager. You are correct, you can just inject the runtime manager into your application.
See the CDI chapter of the documentation how to create producers in order to inject runtimeManager into your bean: Chapter 20. Integration
Or you can just initialize RuntimeManager using ordinary Java, see Chapter 5. Core Engine API.
I have implemented jBPM 6 web application example, you can see there how to use new CDI features:
Btw to declaratively provide a kbase, you have to setup kmodule.xml file correctly. But as I said I would recommend to use runtime manager to run processes.
Hope that helps,