Step 1: In Eclipse, right-click on your project and select the last option "properties".
Step 2: In Project Properties window, select "Java Build Path" in the left pane.
Step 3: Under Java Build Path, select tab "source". It should be selected by default.
Step 4: Click on button "Link Source"
Step 5: Browse it to following location-> COCOS2D-X_INSTALLATION_FOLDER/cocos2dx/platform/android/java/src
In step 5, replace COCOS2D-X_INSTALLATION_FOLDER with your cocos2d-x directory
Step 6: Give any folder name. For example, I gave: Cocos2d-x-source
Step 7: Click Ok.