Instructions
Load your previously saved game of "New Super Mario Bros." on your Nintendo DS.
Tap the world of the level in which you are missing the middle star coin. The bottom screen of your Nintendo DS displays a map view of all the game's worlds, and you can tap the one to which you wish to travel. Wait a moment for Mario to travel to the world.
Use the directional pad to travel to the level in which you are missing the middle star coin. In the upper left hand corner, you can view the star coins you have previously collected in the active level. If this counter displays a coin in the first and third slots, this is a level in which you do not have the middle coin. Press "A" to begin the level.
Go through the level as normal. When you pass by star coins you have previously collected, you will see a dotted outline of where the coin used to be. You can use this as an indicator of where the coin is in the level; if you have passed by a second dotted outline, you have missed the middle coin you are seeking.
Complete the level to complete the collection of the star coin.