Download JavaFX Scene Builder from here(you have to sign in):
https://www.oracle.com/java/technologies/javafxscenebuilder-1x-archive-downloads.html and install it.
Then Open your Installed Netbeans and click on Tools > Options > Java > JavaFX > then Scene Builder Home: browse to : C:\Program Files (x86)\Oracle\JavaFX Scene Builder 1.1 or C:\Program Files (x86)\Oracle\JavaFX Scene Builder 2.0 based on your installation and then click ok. Done.
Enjoy!!