Cards that are Forbidden aren't allowed to be included in the Extra Deck, Deck or Side Deck. While this would normally mean that the cards are unusable, there is a Traditional Format in which you can use Forbidden cards as though they are Limited.
Related ThreadsDrop list for Duelist of Yu-gi-oh! forbidden memories final mod (2011 version) Now finished - last post by @ Feb 27, 2015
Yu-Gi-Oh! Forbidden Memories - Modded Versions F.A.Q - last post by @ Feb 13, 2015
Forbidden Memories: Premium Edition - last post by @ Feb 22, 2003
Yu-Gi-Oh! Forbidden Memories "Soundtrack" - last post by @ Aug 1, 2008
I need passwords for Duelist of the Roses not Forbidden Memories - last post by @ Mar 28, 2003