Normally, PMUs are setup as the "servers" where the openPDC is the "client" - again, normally. The only case where I have ever not known this to be true is with F-NET devices that are typically deployed in locations where "server" style access is impractical.
As such, the PMU should have an IP and port that the openPDC should connect to - in this mode the PMU is the listening (i.e., server) device - and the openPDC will connect to it.
You should be able to ping PMU if it's on the same network (and network tools allow). The port for PMU's is typically 4712.