How to use this unit as a Access Point for an... (by 2 users)

set the second router through the web menus to a different ip address, cant have 2 devices on the same (default) 192.168.0.1 address, try 192.168.0.2 (example only) to stop data collisions
connect by cable secondary router lan port 1 to the primary router lan port 4 (any lan port to any lan port),
drag secondary router upstairs on its cable and install it wherever you want
set two routers on different channels.
set all wep settings on both routers the same.
set all pcs to 'auto channel' select
wireless adaptors in pcs will then select strongest signal at any point and transmit thru that router