Resolved: This issue has been fixed in the 10/16 update.
There's currently a known issue where certain older Mac computers on OSX 10.7 - 10.8 will crash when loading Zwift. This issue is related to a lack of built-in Bluetooth and was introduced with our last update. We're working on a new hotfix update that will resolve the issue, so hang tight!
We appreciate your patience and understanding!