You can test whether Deployment Manager is able to reach a target machine (traversing firewall rules, etc.) by remoting to the Deployment Manager server, and pointing your web browser at http://your-machine-host-name:10301 (or whichever port you chose for machine to listen on).

If you see this error:

System.ServiceModel.Security.MessageSecurityException: An unsecured or incorrectly secured fault was received from the other party. See the inner FaultException for the fault code and detail. ---> System.ServiceModel.FaultException: An error occurred when verifying security for the message.

It can sometimes happen because the clock on one of the two machines is out-of-sync.