Kotlin Android Extensions: How to get a reference to a view in a layout that gets included into another layout?

You should import the included layout.

import android.app.Activity
import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import kotlinx.android.synthetic.main.activity_main.*
import kotlinx.android.synthetic.main.included_layout.* // Here

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        // From activity_main.textView
        textView.text = "text" 

        // From included_layout.textViewInclude
        textViewInclude.text = "textInclude"
    }
}