By default, MS VPL looks in the bin directory within the MSRS installation directory, and creates its list of services from the assemblies that are in that directory.
To add a new service to the list, all that is required is to create a new service assembly. MSRS has tutorial documentation that describes how to do this in C#.
It is also possible to compile a VPL diagram, this creates a single assembly that contains services for each of the activities that comprise the diagram. For a simple diagram, this is just the highest level orchestration diagram. The services created in this way will appear in the services list the next time that MS VPL is started, in our next release there will be a mechanism to update the list without restarting MS VPL