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.)
I found this for me:
For main lib: android.jar: src: sdk/sources/android-19 doc: sdk/docs/reference/
For support lib: android-support-v4.jar: (before this we should add android-support-v4.jar like external jar (Project Properties -> Java Build Path -> Libraries, then in Tab ‘Order and Export’ pull up this library before Android Private Libraries)) src: sdk/extras/android/support/v4/src/java doc: http://developer.android.com/reference/android/support/v4/app/package-summary.html (not sure)
Open SDK Manager-->install source under api(i.e Android 4.0)
if You got No sources attached then click the button, then choose the Directory
e.g: D:\Android\android___sdk\sources\android-17\ ---> this my android sdk location
Depend upon your android installation directory choose accordingly.
First, you need to download them. Use Android SDK Manager -> Android X.Y.Z (API A) -> Sources for Android SDK.
Then, you need to attach the folder with the sources the next time you browse there: /sources/android-A
Previous answers are great but here you have a complete resource by Lars Vogel who address this question. I hope it may be useful as well as it was for me.