There's really only a small number of things I can add here, and I'm afraid they aren't going to be extremely helpful.
-- Yes, the TS3 Starter Pack is the base game plus one EP and an SP. It is not the base game only. I don't know if it is possible to install the base game only from it and not the two packs when going through Origin for the install as it used to be with the disc bundles, it's not clear if this is what @Youngduckho has done.
-- The Mac in question does not have a TS3 supported graphics card/processor combo. There are some players, quite a few in fact, who can never get the game started on that one no matter what they do with sgr and other such files. If Bluebellflora does not have additional advice on how to get things going, then I would have to say no one does. Give her some time to respond to the HQ post, there are time zone differences in play.
-- The experience you are chasing down is (a) not going to be a great one for reasons already outlined, if it can ever work at all, and (b) when the next Mac operating system comes out probably in the fall, it is entirely likely that 32-bit application support will be dropped. That will probably mark the end of TS3 for Mac on all systems staying current with the OS if Apple goes through with that plan.
I really hate to say this because I love TS3 and I love Macs. But the pain involved here is just not worth the return, the game is a mess on Macs to begin with, and even if eventually successful (until the next OS upgrade), this scenario will be repeated with every EP the player tries to add. I'm sorry, but I would suggest asking for refund if no other system is available on which to play.
Edit: Well, of course @BluebellFlora snuck in ten minutes before me and I didn't notice. I need to type faster. But regardless, I would follow whatever advice she has to offer.
