Thanks for the tips, I found it.
Also, I was wondering if anyone else noticed something in 5-1. At the very start of the level, climb up to the upper left corner. You will be able to break the corner open, and it leads to a room with a large question mark and a box. It looks like the box has the "Bridge Construction Set" logo on it. It isn't a secret, so maybe something in this room leads to a Warp Zone (unless it's just for show).
Also, I was wondering if anyone else noticed something in 5-1. At the very start of the level, climb up to the upper left corner. You will be able to break the corner open, and it leads to a room with a large question mark and a box. It looks like the box has the "Bridge Construction Set" logo on it. It isn't a secret, so maybe something in this room leads to a Warp Zone (unless it's just for show).