Yes I tried installing it from both /SEPM/Packages and from the file Symantec_Endpoint_Protection_12.1.6_MP3_All_Clients_EN containing all the packages and I was able to import the Windows clients fine just not the MACOSX client.
For fun I re-downloaded the latest SEPM files and I was able to finally import the MAC client from the /SEPM/Packages folder, I assume the file may have been corrupt? It is all working now!
Thanks for your help!