You can’t just add a <source /> node, if you do this it won’t parse the include.xml and so won’t add the ndll and you’ll get the errors you have.
You need to add it to your haxelibs: haxelib dev openfl-extension-example path/to/openfl-extension-example and use <haxelib name="openfl-extension-example" />.
What else do I have to make different when I have > 5 parameters? I am trying to access the parameters (with for example val_string) and it keeps crashing on me …