If you're using git as your project's scm, add Simple Form as a submodue:
git submodule add https://github.com/makadaw/Simple-Form
Download source code from https://github.com/makadaw/Simple-Form/zipball/master and unzip it to your project root directory.
Add Simple Form project to Project navigator. To do this, drag ITForm.xcodeproj from the Finder to your project navigator panel
Open your project's build settings and add "$(SOURCE_ROOT)/ITForm/ITForm" to Header Search paths.
And copy Resources to your project (if you want to use standart images)