Another option I might add, is to use the project generator.
It has an “import” option which, sometimes, fixes your troubles.
I’ve noticed that some (older) projects on the web have outdated configurations, dependencies, …
You can put the downloaded sketch in your myApps folder and then import it with the project generator.
The project generator then lists all the addons the project needs. If you miss some, then download those and regenerate the project.
O and by the way, there’s a xcode plugin to easily add OF addons to your current project: