I'm pretty sure you need to:
Click on Apple icon -> About This Mac -> More Info -> Network
Look for the router ip. Its under the heading IPv4, opposite router
will look like this 192.168.1.254
enter that number into the address bar in safari on your mac or firefox on your windows machine. It wil take you to your ruouter homepage. It may ask for username and login which is either what you've set or 'admin' for both. From there go into the settings, then into something like "wireless client filter'. there will be a list of wierd looking numbers and letters. you need to add you mac's MAC address to this list. should be able to this autocmatially..
You need to do this for all wireless devices regardless of mac or pc. Hope it helps.