January 12, 2011, 06:03:25 pm
Use the modify object icon to move items to a speciific layer -- Modify Object -> Menu -> Selection -> Layer -> Move selection to Layer... -> (choose layer 1 - 5).
While you can't hide objects by category, you can move those categories to a specific layer, i.e. trees on layer 1, transport on layer 2, etc. Click on the "eye" icon to hide the layer. You can also hide a layer in your movie by using "hide a layer" in scene effects.