If you did the Easy install, you'll never find it, as it's hosted outside of your home directory.
If it's the Advanced One-Click, then it will be in your home directory. Subdomain run in parallel with standard domains. You'll see folders for shortcutworld.com and wiki.shortcutworld.com in the same spot. That is, if you set them both up under the same Owner (FTP user with a home directory).