I got a message from Luigi that says hes lost on a certain planet? Which on is it. I cant seem to find him. I think I have the right planet, but still cant seem to find him. 

The first time he is lost, he is on top of that house on the first planet that you ever went to in the game.

it was hard to find luigi at the bee place.

"The first time he is lost, he is on top of that house on the first planet that you ever went to in the game.
"

Luigi gets lost on 3 planets the Good Egg Galaxy where he is on top of a house that can be done quiet quickly because you can triple jump up there.

Second he is stuck on the Battlerock Galaxy where he is underneath one of the platforms and you have to get a bullet bill to destroy the area he is stuck. This is the most difficult one of the Luigi challenges.

He is also stcuk up a tree in the Honeyhive Galaxy he is quiet easy to find here you have to hit him with the jump and spin attack and he should give you the star then.

Hope this helps

found him. Thanks for the help