If you find that your copy of Don't Starve Together on Mac OS is not launching, first try the steps located in our Don't Starve Together Troubleshooting guide. If those do not help however, try these next few steps and they should resolve your issue:
- Completely exit the Steam Client & fully restart your machine.
- Open Safari and type: steam://flushconfig then press enter.
- Restart your computer one more time.
- Launch Steam and re-test to see if the fix worked.
Please note that you can't do the Steam Flush/reset in a Chrome browser window, it has to be in IE, Safari, or Firefox. Performing this operation will ensure that there are no issues that could be resolved with a new installation of Steam, and is a good place to start any troubleshooting process.
(Steps are directly from: https://support.steampowered.com/kb_article.php?ref=3134-TIAL-4638)