Hi Kezhu,
This is an old post... that's ok, I'm still using both the Synology NAS and SME and they're both working great. There is not much to know connecting the synology to SME is pretty much the same as connecting any machine to your SME Server domain.
On the Synology NAS go to Control Panel > Directory Services.Tick the Join Domain box, type in your local domain name, then enter your DNS server (probably SME server), then apply, it will ask you for a password to join the domain, enter your admin username and password from your SME Server.
You can then apply domain permissions to shares. Most things integrate with the domain username and password, but there are some apps that don't for example, I think the Audio Station requires a local user.
Only other tip I can give you is to remember when prompted for a username and password and you want to use your SME account, the username field will need to be formatted as Domain\username (same as any domain authentication).
Hope that helps.