My problem was solved by following a few steps below.
1) I downloaded the source code via Android SDK Manager. (The sources are downloaded to the source directory located in "path_to_android_sdk/sources/android-xx”). xx is the API level number (15 for 4.0.3). Android Versions and API Levels can be found in the following link for your convenience.
Android Versions and API Levels
2) I zipped the downloaded file. (android-15 in this case)
3) In "Source Not Found" page, I clicked "Attach Source" button and chose the zip file that I archived in step 2.