I presume you have watched the Interactive Training Guides?
http://www.graphisoft.com/education/training_guides/
(P.S. To create a slab automatically using the external walls as a boundary; select the slab tool, adjust the settings and press OK, hold down the spacebar and left click once in the space between the walls. You should now see the slab if you look in the 3d window!)
Hope that helps!