At the end of the day a git submodule is merely a folder on your hard drive. However, from git's "viewpoint" it is recognized as a reference to another repository (at a specific commit) rather than a folder of files.
So if you are using git only (which you are not in this case) then it is a simple yet quite powerful mechanism of nesting projects or including libraries.
To answer your question, I guess you could do it all from the command line, which might be quicker than downloading and unzipping the source code (depending on your preference of course):
cd /path/to/your/project
git submodule add git://github.com/RestKit/RestKit.git RestKit
cd RestKit
# Checkout the current stable branch
git checkout 0.9-stable
# Remove the git repository
rm -fr .git/
From here you can configure your project according to the installation guide