Android: Unable to resolve target android-x

If you get this message then you need to install the relevant Platform version/API level. The number (x) refers to the API level
e.g. unable to resolve target android-5 means you need to install Android 2.0 (API level 5)

To do this in eclipse:

1. Window, Android SDK Manager:
   
2. Select the relevant Platform version
   NB: You may need to select Obsolete to see some versions.
3. Click Install, accept the terms and you should be done.

References

API-Levels