if they work on the channel you viewing press menu /subtitles
if you have sky try sky action they working on that dream elite image but most images are found in the same place:thum:
My experience with HD Content (.mkv) press the button the subtitle will be shown, but unfortunately DVB subtitle has not been broadcast-ed (only hard-code one is available) in my country.
Only subtitle that has been embedded into .mkv will be displayed. Just check it out on PC first, otherwise you have to use some tool to merge a subtitle into the file.