Make sure that the deck you want them to walk on has "walk on" enabled in the sidebar settings (control +b i believe). You can only have one layer, the grid is in 2D so you can never have walkable areas crossing each other vertically. There's a wiki but I'm in my phone so can't really bring it out for you. Should be a quick peek in the guides forum to find it.
Also, place an AI seeder om the deck from root templates, it's a trigger.
