I don't know anything about workflow manager but I do have plain ArcGIS Desktop and Server installed and I cannot find any JTX related assemblies on my system.
My best guess is that the assembly you are looking for is installed with the workflow product itself. Search your system:
cd C:\
dir/s *jtx*.dll
My guess is that it will be on your system in 2 places. One of the places will be the GAC. If it is only in the GAC then copy it out so that you can easily add a reference to it.