you MUST run eclipse as administrator to install plugins under vista or 7!!! right click on the eclipse.exe file (not the shortcut). if you still have problems, try the step below.
one reason this can happen is that you first need to include the release compatible with your version of eclipse:
http://download.eclipse.org/releases/juno (or helios, ganimede, indigo, etc)
in case you are missing the android sdk, download from http://developer.android.com/sdk/index.html
after installing it, you must go inside the folder c:/Program Files (x86)/Android/android-sdk/platforms/ and create a new (empty) folder 'android-7'. then create a folder named temp in c:/Program Files (x86)/Android/android-sdk/ and you will be prompted to download SDK when you start eclipse again (always as administrator)
finally, restart and you should be ok
PS: you must update your eclipse (under HELP) and then exit it. restart in admin mode and open your Android SDK packages (not HELP, the tab on its left [not at my workstation right now, will reedit later]).
without installing basically everything there (some 3 to 12 hours, depending on your connection speed), you will not have a complete environment and some compilation simply will not work.
