iphone 4S 6.0.1 has poor wifi reception upstairs
WiFi networks require two-way communication.  Even though your router may have enough strength to send a signal to that room, whatever wireless client device you want to use in that room also has to have enough strength to send a signal back to the router.

Evidently, the Macbook you tested in the room had enough strength to send a decent signal back to the router.  Yet, the iPhone did not.  Signal effectiveness is a combination of a number of variables, two being antenna configuration, and power.  Your iPhone's WiFi antenna and/or power are inferior to the computer's.

What you could do is either move the wireless router closer to (or into) the room, or else set up another access point to extend your network's wireless reach.

To get a new access point talking to your existing network's router, you'll need to consider using wireless-to-wireless repeating, power line network adapters, or an Ethernet cable &ndash; Ethernet being the best option, if you can somehow run a cable from point A to point B.