If I click on the little triangle to select different instruments, there is nothing in the list (I remember me being able to choose different sounds here):
Click the button on the bottom left of your picture, to expand the preset browser. You should see a few more filter categories like strings, brass, woodwinds…
Check if something is selected there. Reset this filter as well.
HALion isn’t showing me a list of instruments
If the sound browser in Halion or other plugins is blank then you can force the Mediabay database to update by deleting the following folders: (on Mac)
~/Library/Preferences/VSTAudioEngine