It could be that it needs to be set up in the TV via the menu and dropdowns.
It could be that it automatically switches depending on the input.
It would help us if you provided the manufacturer and the model number and the signal source and its model number.
Page 16 of the manual states,
"Auto motion plus ( off / clear / standard / smooth / custom / demo) enhances motion in fast moving scenes) The info on your TV displays the resolution and frequency of the incomming video signal(60hz) this display frequency is not what the TV is displaying by using the auto motion plus function"
Doesn't make a whole lot of sense to me. could have lost something in the translation.
I'd e-mail someone at samsung.com for clarification if needed.
Looks to be automatic, what happens when you play a Bluray disk as the signal source.
All HDTV take 60Hz input and have 60Hz outputs. "120Hz" and "240Hz" refers to internal video processing done by the HDTV's electronics basically for smoother video playback and "improved" video quality.
3D Capable HDTVs are the exception. They are just like 120Hz PC monitors; they take dual 60Hz inputs (120Hz total) and outputs at 120Hz as well. This are the only exception. Here's an example of a 3D capable HDTV, the Samsung PN50C7000YF:
Sorry if I sound a little harsh, but if you are looking for a specific functionality then you gotta do your own research and not solely rely on other people to make your decision.
I don't know what's worse, a salesperson trying to get you to spend more $$$ to get something which doesn't meet your expectations, or a friend to tries to help you out, but in the end didn't know what he or she was talking about and you got burned for it.