Extensions often have dependencies. If you manually copied only one folder, the installation might fail. Always copy the full "Repository" structure provided in the offline installer download.
If the extension doesn't appear, verify that the offline package version matches your installed Altium Designer build (e.g., AD24 files won't work with AD23). altium extension offline install
If you can’t simply hit "update" within the software, here is the step-by-step guide to managing an Altium extension offline installation. Phase 1: Preparing the Offline Installation Files Extensions often have dependencies
Go to (or click the user icon at the top right and select Extensions & Updates). altium extension offline install
Click the button (the gear icon) in the top right corner.