Use these instructions to set up an access point using your DD-WRT router. Don't bother setting it up as an extender, it will be problematic.
http://www.dd-wrt.com/wiki/index.php...s_Access_Point
I have the same set up as you (optik with actiontec and weak wireless upstairs). I set up the DD-WRT router upstairs using these instructions and voila! I can still access my actiontec router to make changes and the Actiontec still does DHCP.
Make sure you follow the instructions carefully and in the right sequence.