This error happens in the following condition:
1. Clean eclipse workspace on a system where Java7 is the default JRE (as shown in windows > prefs > java > installed jres
2. seutp liferay tomcat runtime and choose a locaiton of liferay portal 6.2 tomcat runtime that contains the embedded 1.6 JRE
3. make sure that your workspace preference (windows > prefs > java > compiler ) is set to 1.7 compliance
4. try to create a new portlet project using the created runtime.