Have you configured the IDE to use the correct Java installation (under Tools - Configure Paths)?
Yes, and thank you for your answer.
But my problem was Java 8 SDK.
I suspected that when trying the JavaFX Controls samples, always the same error about no skins.
I installed Java 7 SDK and now everything is working...
But this means that anyone who updates from Java7 to 8 is no more able to use Java FX, at leat these libraries (jFXtrasControls,jar and jFXtrasGauges.jar)?