For a project that uses Carthage without submodules, the Build stage will fail with a Build Failed error.
Click in the toolbar to open the Error Summaries window.
You should see an error as reported by Carthage. Use the menu bar, select Navigate, then Show Checkout Folder In Terminal to open a terminal in the location of the project folder.
Run `$ carthage bootstrap --platform iOS` in the project directory.
Wait for the installation to complete. Click in the toolbar to start Windmill.
Consider using submodules, in which case the Carthage/Checkouts folder should be under source control.