Floydy - if it's the same as my router, click on "Home Network", then "Interfaces" underneath it.
Then click on "WLAN: <your-SSID-name>"
Then at the bottom of that next page, is there a "Search for Wireless devices" link?
On mine, this link will put the speedtouch into registration mode for 60 seconds or something, during which time you can join a wireless client up to the network. After this point it locks itself again.
As rizla said, though, it's very easy to change the MAC address of a network card to pretend to be a different one (called spoofing), and if someone is intent enough to get past your WPA encryption key, spoofing a MAC address is the easy part.
However unless someone's got a reason for hacking into your particular wireless network, WPA with a long encryption key is enough to deter the casual 'wardriver' from trying to get some free internet access for a few minutes!
