In order to unlock a level in level editor, you must rank a C or higher in it. If you rank a B or higher, you get a set of items corresponding to that level that can be used in any place.
To capture the third Pink Elebit, do the following:
When:
During the game
How:
Mission 1 Appears if you clear the mission in 5 minutes or under.
Mission 10 Appears if you clear the mission with an 80% durability.
Mission 5 Appears if you clear the mission with fewer than 3 loud noises.