That said, I cannot seem to add my company's Mac client install package (which already exists in the "Install Packages" area, though it is listed as version 11.0.7101.0236 for Mac, whereas the Windows versions are 12.1.1000.157) to the "Clients" tab...
To import newer MAC install packages to the SEPM console just follow the below articles:
http://www.symantec.com/docs/TECH131675
http://www.symantec.com/docs/TECH183884
...the MAC packages (.info file) will be available on the SEP installation media as indicated in one of the above articles -> this way you can provide SEPM with newest version of SEP for MAC.
Unfortunately it is not possible to use the Autoupgrade SEPM feature to assign the MAC packages for groups [Clients Tab] (this option works only for windows clients) - see here more in FAQ for MAC:
http://www.symantec.com/docs/TECH134203
Q. What about upgrading SEP for Mac to a newer version? Can I use Upgrade Groups with Package (auto-upgrade)?
A. Unfortunately, you must export a client package for the new version then install or deploy as you would a new installation; it is not possible to use the Upgrade Groups with Package wizard (auto-upgrade) to migrate Macintosh clients up to a higher build. However, you can install the new version directly over the old without uninstalling first.
...nor can I include the Macintosh as an included platform on the LiveUpdate tab of the Site Properties.
The Admin-> Server-> Local Site -> Site Properties -> Liveupdate -> these settings are only for SEPM downloads from Symantec Internet Servers.
To inlude MAC downloads here do the following:
1. The Admin-> Server-> Local Site -> Site Properties -> Liveupdate -> Platforms to download -> Add MAC
2. In thge Liveupdate policy assigned to MAC clients, edit the MAC Settings -> Server Setting and check the use if the default Symantec Liveupdate Server for retrieving the updates (you can set here as well schedule for downloads here)