Do you mean that it won’t show up as an option when you choose an instrument to load in the rack slot?
HSO won’t show up in your VSTi rack because it is not a standalone instrument. It is only a library that shows up in HALion, HALion Sonic, or HALion Sonic SE.
In the VSTi rack, select one of those and then look for the HSO library within those instruments.
HTH
J.L.