Hi takaeku!
For this Sibelius version the "Vienna Symphonic Library" house style should be installed automatically by the VSL Soundset Installer.
Could it be that you have run the VSL Soundset Installer with a different account than the one you are using Sibelius with? Please run it with the account that you are using Sibelius with.
If this doesn't help, do the sound sets get installed, or are they missing too?
Best,
Andi
Vienna Symphonic Library