The UML functionality is only available to Professional Subscriptions, if you have a professional subscription, you should have the features unlocked in your install and can follow steps on how to use it here: http://www.myeclipseide.com/images/tutorials/quickstarts/uml/
I would suggest you upgrade to atleast MyEclipse 4.1.1 as there were a lot of bug fixes that went into UML by that release that can help make the experience smoother.