1. Go to Tools > Extensions
2. Look for your extension’s ID. Note this down. You’ll need it later.
3. Open up Finder, then go to Go > Go to Folder …
4. Type Library and click Go.
It’s a hidden directory on the lastest version of OS X.
5. Navigate to Application Support > Google > Chrome > Default > Extensions
6. Look for a folder with the same extension ID, go there and now you have the source code for the extension!