After installing ADT, I used SDK Manager to download the source codes for the latest android API. Now when I run the Eclipse version that was provided with the ADT, I still can\
After downloading the Sourcecode by useing the "Android SDK Manager", you have to change the settings in your eclipse-porject.
At first, just have a look where the Sourcecode has been downloaded. In my case (useing Windows 8) its here: "C:\adt-bundle-windows-x86_64-20130219\sdk\sources\android-17"
And now you can see/read the Android-Sourcecode. (But only in the project which you have choosen before.)