(a) Open the class and select 'Link to Editor' action on the explorer.The explorer should get itself drill down to the jar.
(b) If you have the UI, want to know the contributing class and the plug-in , just press alt+shft+f1 for the plug-in spy which helps
you to find out the class and the plug-in.
Be Objectively Oriented.Explore the power of OOPs.
My Blog, Eclipse EMF Query committer.
JBoss Tattletale - provides way more that the class location, also tells you exactly which JAR files your code depends on.
Isocra Jar Finder - tells you which JAR file contains a class given a regular expression. I use this one constantly (though I did modify it, the original has a few quirks I disliked)