android target api not visible

i have installed android sdk. i have also downloaded and installed the adt plugin in my eclipse ide. now when i go to preferences to set the path 开发者_如何学运维for android sdk, when i set the path and then press apply-- automatically the target api should be listed below. But they are not being listed below for some reason. what could be the problem? thank you in advance

Within eclipse, go to Window > Android SDK and AVD Manager then choose Installed Packages from the left hand side. Do you see things like SDK Platform Android 2.3, API 9, revision 1 listed? If not then you need to download them. Select Available Packages then expand Android Repository then select the SDK versions you want to download.