This bit walks through Ableton Live's Plugins tab, the settings hub for hooking up third-party instruments and effects. You can enable or disable Audio Units, point Live to your system VST folders, and set up a custom folder if you keep plugins somewhere non-standard.
If a plugin isn't showing up in your browser after installing it, Live won't always catch it automatically. Triggering a rescan from this tab is the fix, a quick step worth knowing before you assume something installed wrong.