Here is a project which demonstrates a business process waiting on an intermediate message event that is triggered by an external service.
It is a simple voting process (for/against) where the business process is started from the console and will wait at the intermediate message event for a message with a matching Proposition string. You can use the wsdl in the build folder to generate a test SOAPUI client to represent the external service. The external service receives a reply with the result of the two input messages to the process.

This may also be a clearer view of Explicit Correlation than the existing sample.
The Data Editor was used to create the correlate functionality by
1. correlation: $prop-corr
2. Property to be used in the correlation this:Proposition within namespace this="http://thisdiagram/Complexvariable"
3. mapping incoming message "receive" to correlation which also inititiates the business process instance
4. mapping incoming message "from svc" to correlation which looks for existing process instance


David