We've had some players with have start up issues with the mac. If you are having issues, can I have you try resetting to defaults, checking your savestate, and if that doesn't work try disabling your antivirus?
1) Right-click on the game in your Steam library, go to Properties
2) Select "Set Launch Options"
3) Enter the text "--resetToDefaults" (no quotes) and hit OK
4) Launch the game
5) After you’ve done that remove the --resetToDefaults"
If that doesn't work.
There might be an issue with your mac’s savestates. You need to delete your save state for guns of Icarus.
Open Finder and select the Go menu, while in the Go menu, hold down the option key and select the Library menu which appears.
Once you are in the Library folder, look for a folder named “Saved Application State” and open it.
Inside of the “Saved Application State” folder find the applications folder named “unity.Muse Games.GunsOfIcarusOnline.savedState” and delete the directory by moving it to the trash.
Try opening Guns of Icarus Online
and if that doesn't work make sure Guns of Icarus is allowed by your antivirus or is disabled. This is especially important if you have Sophos.
Let us know if that solves your issue. If not feel free to shoot us an email at firstname.lastname@example.org.