I baught Mario Galaxy for my nephews for Christmas, but I can't play in coop mod, it says that I need to press A with the second controler but it just won't work.
You realize it's just a second star pointer that can do a few things, right? It should be a yellow pointer on the screen. The second blue light on the co-op remote should be on. If it's not, even after pressing A, try manually connecting it in the home menu screen.
Make sure the second Wiimote is synced to your console, too.
I really don't think there's more to it than that.