It's been a while but I remember a host of back-and-forth customer-service emails with the result being that the best way to install it is through the Steinberg Download Assistant software. I can't say why for sure but it didn't install where I would normally want/specify my 64-bit VST dll files to go. Here's where the dll lives on my system:

C:\Program Files\Common Files\Steinberg\Shared Components\HALion Sonic SE


I hope that helps.

Paj
8^)

BTW: A freebie instrument for Halion Sonic SE3:
https://sonicatoms.com/novel-piano/