You simply need to specify the right port in the tracker address, 8945:
If several trackers are used, make sure to specify the correct number of trackers in the MiddleVR’s VRPN configuration.
Known bug: Until MiddleVR 1.2.2, a bug exists which might cause an impossibility to set the permutation axis correctly. Contact support for a fix for MiddleVR 1.2.2. This is fixed with later versions.
If needed, here is how to Troubleshoot VRPN.