Ok, was this router supplied by your ISP (and who is your ISP)? If so, have they nobbled the WebAdmin for a reason - they don't want you to fiddle with it? Do their T&C's only allow a single machine?
Are you referring to the MAC address filter for the wireless authorisation - this
page suggests this router doesn't support MAC address filtering.
If the router is your own and doesn't work correctly, I would either return/RMA it or buy a cheap router to replace it.