How old? I recall seeing a post that older Samsung TVs are not longer supported. I'd guess if the app isn't available then yours may be too old.
My Sammy is roughly 10 years old and I don't think it ever had an available Roku app.
If you can cast to your TV you could view therokuchannel.com via the web browser on a computer/tablet/phone and cast it to your TV.
Or you could install an actual Roku on an HDMI port on the set and stream The Roku Channel and lots of other stuff that way.