Download the zip and leave it zipped — Blender wants the archive, not a folder. In Blender open Edit, then Preferences, then Add-ons. Click the arrow button in the top right corner of that window and choose Install from Disk, then pick the zip you downloaded. On Blender 4.0 and 4.1 the same button is called simply Install. Tick the checkbox next to the add-on name to switch it on. Press N in the 3D Viewport and the Rig tab appears in the sidebar.