you probably add the org.knime.ext.weka_3.7 plugin as a dependency to yours? Unfortunately it doesn't work this way because the weka-classes are not re-exported by the org.knime.weka_3.7-plugin.
I opened an issue to add Weka to the KNIME's target platform (i.e. wrapped as OSGi-plugin) to make it quite easy in the future to use weka dependencies.
If you don't understand what the heck I am talking about, no worries. For now, you can just download the weka library (a jar-file) and add it to the classpath of your plugin (e.g. copy it into a lib-folder within your plugin, add it to the MANIFEST.MF-Bundle-ClassPath and update the project's classpath settings ...).