Q: how to enable subtitles?

Try this:

While playing the video.

Up button on the remote

Up button (again)

You may see the description, and below that something like: Hold center/select button for options

I can't really tell you about TV Shows on iTunes, since they're not available on iTunes here in Mexico.

As for movies, if you downloaded them yourself from other place than the iTunes Store, I'm not sure you can simply enable subtitles like that.

Thank you

Up arrow and holding down select worked fine for me.

Hold remote button and wait for options to appear