Patrick, what you need to do to use rules within the process is following:
- define your rule within same package as the process and assign it with rule flow group attribute
- add business rule task into the process and fill in the rule flow group attribute of the task to the same value as you defined in the drl
- depending on what you are about to reason on you need to insert facts into the session - for simplicity you can use script task before the rule task to do so
that should evaluate the rules for given rule flow group when process instance is executed. Please note that in rule's consequence you need to do what ever is required to make the decision, like set process variable etc.
Alternatively you can use drools as a language of gateway's sequence flow condition and then use LHS from rule directly inside the process.
HTH