Head back to the area with all the really tall pipes towards the beginning of the Underwhere section, the bit where you had to 3d flip and jump over pits/spikes, all that.
At the beginning of that area (by the door or pipe to get in, I honestly don't remember which it is) there are two very very tall pipes to the far left, use Luigi and his super jump (hold the down button) to jump up on those pipes. From there jump up to the next floor and continue right.
