How about installing either a repeater in that room or maybe connecting up a power line unit? Seems like you need to get a source signal in there instead of trying to rely on the main host router trying to penetrate thru the concrete.
I took my DIR-655 router and turned it into an AP router with your guide.
http://forums.dlink.com/index.php?topic=40856.0
Seems to work splendidly but I'm wondering if there's any extra securities I can put into the AP router. I have MAC address filters on my DIR-636L and nothing on the DIR-655. Does the MAC address filters from my host router take into effect? Or the only security I have on the AP router is the wifi security?