The extension adding process should be straightforward, and you’re already in the right place for it, so the actual question here is what’s not working as expected.
If you aren’t already on it, could you try updating to the latest stable BlueOS version? Currently that’s 1.1.1.
If you’re already on the latest version then I recommend checking the notifications in the top right to see whether there’s anything relevant listed, and try running an internet speed test in the Network Test page to confirm there’s a stable internet connection. If that doesn’t help you to resolve the issue then please provide some system logs so we can help to diagnose what’s going wrong