Step 1 : Change the trust level in the Package Center and allow Beta
Log into your NAS as administrator and go to Main Menu → Package Center → Settings and set Trust Level to "Any publisher".
Then, go to Beta tab, and check "Yes,I want to see beta versions!".
Step 2 : Add Zappiti Packages Sources
In the Package Sources tab, click Add, type "Zappiti" as Name, and http://using.zappiti.com/synology/beta as Location.