Download MetaRepository_20160513_arm_013.pkg , rename to "MetaRepository.zpkg" , and put the text file in \\<NAS>\admin\zy-pkgs\. Then in the package menu press on the 'Retreive List From Internet' button. (On firmware 5.20+ this is the 'Cycle icon' in 'App Center->Browse->All Apps'). Only the MetaRepository package should show up. Install it, and re-request the list. You should see the (MetaRepository). If you still see only MetaRepository, disable it, enable it again, and retry.
After the install metarepository, click on them, install plex