I do believe I can help!
I have a Mac as well, and it doesn't put up with it without crashing while you play it. Have you tried to reinstall the game?
This has happened to me, and as you can see it has happened to iValentien. She restarted her computer and it still wouldn't work, when she redone loaded it, it then worked. Try to redownload it, and tell me what happens after.
Hope this Helped!
Well, first off, what Macbook do you have and what generation?
Since I don't really experienced this problem as a Macbook Pro user (one of the latest generations), it might be a problem your computer has.
As I read you understand where to put them, it's all fine. Maybe you can change some of your settings in you pop-up menu before starting the game.
This thing:
(http://i62BannedImageSite/2lw3m2t.png)
I have my settings to this. And it works all fine with the mods I got in-game. You may also look at your game settings itself, under the Interface or Video panel? Can do the trick.
I DO recognize the problem myself that an error may occur 'sometimes' because the game doesn't run properly for some reason. It shows in a most likely gray pop-up when your game is starting to freeze. But I heard many about that. It doesn't necessarily have to be the mods/massmarkings you installed. But it could be, since it sounds very likeable to be so. Notice me saying sometimes in apostrophes because the 'for a little' you mention might vary, I assume? Meaning to say that the error pops up randomly about an hour or 2-3 or within a 15 minutes.
My very last option; look at your files once again if they don't happen to just have misplaced somehow, even though you are totally sure. Also look for double files maybe!
Small other notice; Apple computers are known not to run every game so well. Hence I think an update is needed, but due to long absence of the coder/creator of the game, known as KovuLKD, it is not possible just yet.