We are keenly interested in developing 'onboard' Apple Music, but unfortunately, permission to do this from Apple has not yet been forthcoming. As such, Apple Music Lossless is not currently officially supported on our range of Network players via USB audio or other means.
Please accept my apologies for any inconvenience or disappointment this may cause.
With regard to the USB input - A Mac will need to connect to the large USB port on the rear of the 851N - this port is for USB audio, whereas the others are for portable storage. We have had some reports of users finding success with connecting a Mac to the USB port in this way and playing lossless Apple Music files, but as this has not been officially tested we can not guarantee compatibility in this way over USB, and even if it does work there may be some idiosyncrasies in the way the implementation works and files or track is displayed.
It is also worth mentioning that USB audio is a protocol that allows computers to talk to sound cards, amplifiers and other audio peripherals. An iPad or iPhone will lack the requisite hardware for USB audio functionality. As such, it is not possible to connect an iPad or iPhone over USB audio to the unit in this way.