Have you definitely downloaded and installed from the link in the other thread, even though you may already have Halion running ok in Cubase.
After you have installed from link in other thread (as this seems to make the thing visible)
Try here to find the Halion Sonic.dll,
C:\Program Files\Common Files\Steinberg\Shared Components\HALion Sonic SE
if no luck use ultra search program to do a quick search on your local drive, its fast, you can get it here.
https://www.jam-software.com/ultrasearch/After you do the search take note of the location or locations that you have the.dll, and then try each location in turn in the plugin options in biab, until you get one that works.