Go into the build path for your project, select libraries, and find the JDK entry there. Open that up and one of the entries in the subtree should be "Source attachment". Edit that to point to the src.zip file in your JDK install directory.
Joined: Nov 09, 2008
The first 6 jars (resources.jar thru jfr.jar) already have Source attachment set to C:/Program Files/Java/jdk1.7.0_05/src.zip which I think is correct. The remaining 7 jars don't have anything, but I think that's normal.