When you do a search for "Super Stardust HD" in the PlayStation Store, does the game show up as being purchased? If you indeed purchased the game under the same account that you're using to find your copy then the game will show up as purchased in the records. If not, could it be that you purchased the game on a different account?
Thanks, but there is nothing listed on the downloaded games list, it only shows that I downloaded the demo, which I did at one time but later bought the full game, perhaps I'm looking in the wrong area
Did you play the demo for a while before you bought the unlock?
Maybe the unlock is just not near the demo in your DL list because of a difference in time between getting the demo and buying the unlock?
You also got many other posts of solid advice to follow here, in the end if you can't get it to work use that support link that was posted!
If you already bought it, when you go to the PS Store, you should be able to just download it again (instead of hunting for it in your Download List). Since you downloaded the Demo, then much later bought it, you need to find both pieces of the puzzle to play it (download the Demo, and the game unlock file itself). Because the Download List is in chronological order, they will likely not be near each other in the list.
Make sense? I'd just hit the PS Store and re-download it from there. Simple!