Here we'll post some useful guides for doing things, and workarounds for known issues.
The Mac version of Mannequin is currently unsigned (because I don't have the money to pay that $99 Developer Program subscription fee), so you'll need to disable Gatekeeper in order to run Mannequin in your Mac.
I'm quite sure there's nothing wrong with the app, but I should remind you that disabling Gatekeeper increases the security risk of your Mac. Please do this with care, and remember to re-activate Gatekeeper again after running Mannequin for the first time. Or, just wait for the signed version if you have the patience (hopefully it will be available around September-October).