Switch to Luigi:
To swich from Mario to Luigi, press Select.
(supplied by: cub)
Get a lot of points:
In 5-2 where you first enter there is a koopa. Shoot the koopa to your left and chase it, if done correctly you should get an extra life. Keep doing this and you should get a lot of points. Then with a lot of patience you should a score high enough to open luigi world (over 300,000). (supplied by: cam14)