Problem
Description Resource Path Location Type The App Engine SDK 'App Engine' on the project's build path is not valid. Mail Unknown Google App Engine Problem
SDK directory '/Applications/eclipse/plugins/...334/appengine-java-sdk-1.2.0' does not exist
Solution
Reinstall GWT and GAE in Eclipse (by uninstalling them, and then installing; and don't forget to restart eclipse each time)
Then go to the project properts and configure google SDK's once again
To create eclipse project make sure to run from the samples applications before you try to import it into eclipse
../../webAppCreator -ignore com.google.gwt.sample.mail.Mail