Well, typically you shouldn't need to build spring-flex-core yourself at all, unless you are trying to make changes to it. I would expect the more common case to be only importing and building the Test Drive, as that's the one we try to ensure works out-of-the-box.
That said, if you really do need to build spring-flex-core, then the easiest thing to do is go one level up (to the main spring-flex directory) and execute 'mvn install' from there. This will ensure the parent POM gets published to your local repo as well.
Alternatively, you can just point to the Spring milestones maven repo in your settings.xml.
Staff Engineer, Web Products Team