If you are not able to use the controls using the above instructions, please submit a ticket and include the information from the App Info screen so we can troubleshoot: How to find the App Info screen on a Smart TV
To fast forward on the new Samsung TV app you will need to pause the episode you are watching so that it brings up the tool bar for you:
You will then need to move your cursor down to the scroll bar by pressing down on your remote. You will see the area that you are in as it will be highlighted (above image shows the pause sign highlighted).
The below image is after pressing down on the remote and you can see the circle on the scroll bar is now highlighted.
Once you are on the scroll bar at the bottom of the screen you can then fast forward and rewind using your remote control.