They say a picture speaks 1000 words. Thank you very much for posting that screenshot
The correct answer for your question is:
Close and restart Synaptic.
The 'smplayer' entry you are seeing on the left hand column is (as correctly pointed out by poster above) your search term. It is not an entry for software in Synaptic.
By default, Synaptic will remember your searches conducted during that session so that in case you want to do it again, it is just a question of one-click.
If you still want to install SMPlayer, please give us some more details regarding your problem.
After searching for SMPlayer, what EXACTLY did you do?
What happened then?
What was the error message you got if any?