Once you have the super boots or the ultra boots go to the main gate in Toad Town and you should see a pipe leading to your house. Go down the pipe and go into your house. Go all the way left and you will see a floorboard that looks weird. Ground poung it and you should be in a secret basemeant. On a table you will see a dairy. If you read it you'll find out what Luigi has written over the past few days.