java.lang.NoClassDefFoundError

If you get this error it is probably because the necessary jar files aren't on your build path.

(in Eclipse)
(1) Right-click your project -> Build Path... -> Configure Build Path
(2) Click the Libraries tab and click "Add JARs..." if the jar file is local to the project directory or else "Add External JARs..." if the jar file is outside the project directory.
(3) If necessary, associate a native library by opening the file explorer tab on the left side of the jar file once it is listed within the Libraries tab. You can also specify the Javadoc location for the jar file if it was created without the source.