Stack trace when installing external Essentials feature

When installing the Training: 1... plugin added to the library through

<pluginRepositories>file:///Users/bleunis/Documents/temp/training_features_descriptor.json</pluginRepositories>

we get a stack trace:

[INFO] [talledLocalContainer] 01.05.2015 09:17:55 WARN http-nio-8080-exec-2 [PluginParameterServiceFactory.getParameterService:49] Problem retrieving parameter service class org.onehippo.cms7.training.essentials.webdevelopment.WebdevelopmentParameterService for plugin Training: 1 - Web Development Module.
[INFO] [talledLocalContainer] java.lang.ClassNotFoundException: org.onehippo.cms7.training.essentials.webdevelopment.WebdevelopmentParameterService
[INFO] [talledLocalContainer] at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1305)
[INFO] [talledLocalContainer] at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1157)
[INFO] [talledLocalContainer] at java.lang.Class.forName0(Native Method)
[INFO] [talledLocalContainer] at java.lang.Class.forName(Class.java:260)
[INFO] [talledLocalContainer] at org.onehippo.cms7.essentials.plugin.PluginParameterServiceFactory.getParameterService(PluginParameterServiceFactory.java:42)
[INFO] [talledLocalContainer] at org.onehippo.cms7.essentials.rest.PluginResource.autoSetupIfPossible(PluginResource.java:312)
[INFO] [talledLocalContainer] at org.onehippo.cms7.essentials.rest.PluginResource.autoSetupPlugins(PluginResource.java:255)
[INFO] [talledLocalContainer] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)