Drools Eclipse plugin can't recognize third party classes

Whenever I use classes from any third party jar files (not in JRE System Library and Drools Library), I got error messages in Eclipse IDE.

At the place where I import the class, the error message is "Only a type can be imported. ... resolves to a package".

At the place where I use the class, the error message is "... cannot be resolved to a type".

However, I have included the jar file in classpath. The project can also be executed successfully even with the error message in IDE.

Below is my development environment:

• OS: Mac OS X 10.9.2
• Eclipse Kepler
• Drool Eclipse Plugin - JBoss Drools Core 6.0.1Final. (I don't have problem with Drools Core 5.6.0Final)
• Java SE 1.7
• Drools Runtime 6.0.1