How to update PMM Agent if its installer can't be downloaded from Distribution Points?
Please create a Task Sequence having a single step Execute Script and specify this script there. Don't forget to replace %YOUR_PMM_PROXY_URL_HERE% with you PMM Proxy URL.
Then deploy this Task Sequence to the desired Device Collection (All Mac OS X Systems by default) as Required. Macs will download the PMM Agent installer from PMM Proxy directly and update their PMM Agents.