A wiki page tells you how to do that. So, go ahead enjoy JDT's java 7 features. Please file any bugs, or enhancement ideas you find/have in the compiler, content assist, code select and java search here with the [1.7] tag in the "Summary". This is still a work in progress and even though a lot of work has been done and several new compiler and UI options have been introduced for java 7, we're still working on new features and testing/improving the ones already implemented. More details about the state of the art are available on http://wiki.eclipse.org/JDT_Core/Java7 and http://wiki.eclipse.org/JDT_UI/Java7.
Note: This is an implementation of an early-draft specification developed under the Java Community Process (JCP) and is made available for testing and evaluation purposes only. The code is not compatible with any specification of the JCP.