Could you verify that you are running 5436 on both master and slave by looking at the VE Pro Server window title and by clicking on the top part of the VE Pro Server Interface plugin (where it says "Vienna Ensemble Pro Server Interface")? 5436 is not able to discover older version slaves (and older version is not able to discover 5436 as well).
If you are indeed running the same version on both systems, did you have to configure a firewall somewhere to use VE Pro before? Two additional network ports are used in this version, namely 6476 and 6477. You might want to add these to the exceptions in your firewall (the software firewalls in Windows or Mac OS X should do this automatically though).