hi,
i can key in the mac address and set it to "Accept" at the Mac Filter Mode. But what do I need key in to reject all others?

Yes. That is just your PPPoE credentials. A good real world example will be logging into your facebook acc using a new laptop/PC from old laptop/PC. So ya, you can save your PPPoE credentials to be used in your new router.

EDIT: Just makesure to keep the credentials for future use, just as a safety precaution. 

Refer to your unifi installation document (blue paper) or you can request from unifi itself. Just say you accidentally reset your router and need the credentials to set up the router back again. Or another simple method is to gain your PPPoE username and password through the wireless router admin panel (also you can refer to this thread - credit to bro soonwai, just in case) and its pretty simple if you follow the right guide especially if you're a novice user. Sorry for the delayed response mate.