That's strange. I just tested it on my own machine and it's definitely working. If it's not a bother, can you tell me more about your MacOS version (High Sierra? Mojave?) and the method of your installation (via the itch.io desktop app or by downloading the zip file via the itch.io website) ?
For your information, in the meanwhile you can install the app using itch.io's official desktop app. It's quite a bit of a hassle initially, but it has the benefits of enabling auto-update for Mannequin whenever the itch app is running. Mannequin itself has no DRM, so you can just launch it after installing without keeping the itch app running at all times.