Be aware this feature is about connecting your own Lightning node to your account on getalby.com/login. It is still experimental. If your node is not well connected and does not have enough incoming liquidity, payments will fail.
This feature is not yet connected to the Alby browser extension. You need to connect your node on getalby.com/login and the browser extension separately.
However, if you open an Alby Lightning wallet on getalby.com/lightning-address, you can use it for the Alby browser extension, too.