You can search through the list or simply type in your NAS model.
In the Synology download center, select "Desktop" and download the Synology Assistant.
Once you've downloaded the right app for your device, open it and run search. The app should automatically start searching when you open it, however in some cases you'll need to manually start the search. The most common reason why your Synology NAS fails to appear is the security settings on your Antivirus. Avast Antivirus is especially problematic. Just temporarily pause your antivirus while you run the search again.
Once your NAS has appeared in the list, you just need to click "Connect" and a new browser tab will automatically open, directing you to the Synology NAS dashboard!