In configuring the components for MPOS you might get stuck sometime on the Test Connection step of the Async Client when testing the connectivity to the Async Server. Do not forget the types of each component in the equation here and act accordingly.
One reason for the components not properly communicating can be the following error (do not forget to look into the Event Viewer whenever you stumble across an error):
"WebHost failed to process a request.
Sender Information: System.ServiceModel.ServiceHostingEnvironment+HostingManager/11429296
Exception: System.ServiceModel.ServiceActivationException: The service '/AsyncServer/UploadService.svc' cannot be activated due to an exception during compilation. The exception message is: Memory gates checking failed because the free memory (840810496 bytes) is less than 5% of total memory. As a result, the service will not be available for incoming requests. To resolve this, either reduce the load on the machine or adjust the value of minFreeMemoryPercentageToActivateService on the serviceHostingEnvironment config element.".
The error is pretty self explanatory. In this case, I've just ran out of memory (just you did if you're in the same situation), so clean up some and retry to Test Connection.