Here's the solution you've all been waiting for to play ACB on mac:

1. Go into finder
2. Do cmd+shift+g
3. Copy and paste: ~/Library/Application Support/steam/SteamApps/common/
4. Go into Assassins Creed Brotherhood folder
5. Right click on the application and click "show package contents"
6. Go into contents then MacOS
7. Launch the acbmp_sf file and press ESCAPE and then ENTER once application has launched

I hope this helps everyone with macs! 

After doing some research I was able to come up with the solution to allow the game to launch through steam and then allow you to access ALL of the features!

WARNING: YOU WILL REQUIRE ANOTHER ASSASSIN'S CREED GAME IN ORDER FOR THIS TO WORK FOR EXAMPLE AC2

Just follow these steps:

1. Do the steps from the solution above from steps 1-4
2. In the ACB folder you will find another folder called Ubisoft. Open the folder.
3. Inside you will then find the Ubisoft game launcher. Move it into the trash.
4. Open a new window in finder do cmd+shift+g.
5. Copy and paste: ~/Library/Application Support/steam/SteamApps/common/ (again)
6. Then navigate to your other Assassin's Creed game.
7. In that AC folder open the Ubisoft folder.
8. Now simply copy that ubisoft launcher and paste it where the old one was.
9. Now open the game through steam and enjoy the full experience of ACB and get your moneys worth!

Please rate up this guide if you found it useful!