Besides the info that you are told of how to resolve your issue of about how to log-in to the router..
#1 With MAC Address filtering there are the following problems/issues:
a) Regardless of the type of encryption (Wireless encryption types include WEP, WPA, WPA2) MAC Address(es) are always sent in the clear!
b) Not only it is possible to spoof/clone a MAC address in a NAT router..
For example as noted as shown at http://ui.linksys.com/files/WRT54G/v8/8.00.0/WanMAC.htm
..it is also possible to spoof/clone the MAC Address on a computer!
c) If an attacker user does not know how to spoof/clone a MAC address without encryption: While they can not use your network, they can see what you are doing!
#2 So just use the highest level of wireless encryption that all of your gear supports.