Halion Symphonic orchestra is just a “library” of sound programs that you load in Halion Sonic, Halion Sonic SE, or Halion 4 like any other program. You can find the programs more easily by filtering out just the HSO programs. When you go to load a program into a slot (I am talking about Halion Sonic SE here) in the “filters” window click the “attributes” box. Then on the left column select the drop down and go to “audio-assets” -> “library name”. Now, in the left column you can select the different libraries you want to filter by. If you have HSO installed, it should show up as a library filter option.
p.s. Not sure about Halion Sonic or Halion 4 but there should be a “library” filter that works similarly.