Jabber 12 is still in the Early Adopter Program. I believe you need to join the EAP to get the COP file.
All the clients are now available via cisco.com and users with Android & Apple devices are getting the updates automatically.
Ah! I was unaware that Jabber 12 had been released.
The COP file is part of the Cisco Jabber for Windows Admin download from the Cisco Software download site. The specific COP file you are looking for is: cmterm-jabber-desktop-eight-Lines.k3.cop.sgn
It is in the "CiscoJabber-Admin-ffr.12-0-0.zip", which can be downloaded from the "Jabber Software-12.0(0)" area.
Jabber Multiline is supported on Macs. In addition to the COP file, you will need Jabber 12 for Mac. Are you registering direct to CUCM or are you going through Expressway?
It's in a box folder and the documentation provides the link