Try browsing to one of the support classes. The source attachment should work now.
Worked perfectly in my case.
One thing to note: if src is not an absolute path, it will be resolved starting in the parent directory of the JAR file. Taking support library as an example - if src=support/src, ADT will assume that the class sources are located in libs/support/src.
Short description of this feature written by its author can be found here.
If anyone is interested in how exactly this .properties file is processed, I recommend reading patch set #4, esp. changes in eclipse/plugins/com.android.ide.eclipse.adt/src/com/android/ide/eclipse/adt/ internal/project/LibraryClasspathContainerInitializer.java :)
Edit
Please also see a fine comment by WindRider about adding sources properly in a project with multiple referenced libraries.