I've installed Houdini 16.5.439 and Unity 2018.1.0f2.
Every time I drag a HDA into Unity scene, It just throw an error like that:

Those errors lead me to believe that the plug-in might NOT have been installed properly (looks like everything is inside the Editor folder which is incorrect). Did you import the plug-in package via Assets->Import Package? Or perhaps accidentally moved the other folders into the Editor folder?

I just tested on Unity 2018.1f with H16.5.466 and its working correctly.

Thanks for your reply.
I create a new Unity project and import Houdini Engine V2 and it works just fine.
Maybe it's just some problems with my previous Unity project.
Thanks again, the new version of the plugin is cool.