Try following the steps here, but put the navigational script on a cube that you want to walk on your plane: http://docwiki.unity3d.com/uploads/Main/Scripting%20Tutorial.pdf
answered Dec 24 '10 at 08:49 AM
Try using the terrain editor. In the Scene Editor go to Terrain>Create Terrain and play around with the editing tools. (Linky: http://unity3d.com/support/documentation/Components/script-Terrain.html) If you want to be able to make something walk around, try asking for a 1st person or 3rd person tutorial or just Google Unity 1st/3rd Person Tutorial. Good Luck!
answered Dec 24 '10 at 09:06 AM