I would make sure your Wikitude plugin header and source files are in the Plugins folder in your Xcode project. I would then verify what the interface name of this plugin is.
<feature name="WTWikitudePlugin">
<param name="ios-package" value="WTWikitudePlugin"/>
</feature>
Particulary, you want to pair up the value in the <feature>
tag to the interface name in your header/source file. This is how config.xml links the plugins. The JavaScript file will call the plugin using the "name" property - so this is how Cordova links the JavaScript to the config.xml, to the native library.