You will have to provide more information or a screenshot since there are several reasons that could cause such a problem (e.g. you may miss a JAR file from the imported library, or you have not imported the necessary object in java class file). However, a very good tutorial about Weka and Netbeans can be found on the link attached below.

http://www.cs.umb.edu/~ding/history/480_697_spring_2013/homework/WekaJavaAPITutorial.pdf

follow the link below

https://weka.wikispaces.com/Netbeans+6.0+(weka-src.jar)

I assume (I may be wrong) you managed to had the jar to your project libraries in the project properties, you can use Weka classes but not get weka classes to appear in the deploy version if that is the case there is a packaging option to include dependent jars in the build. If this was not your problem please discard my answer.